Add loadable agents to the backend

[clalancette]

We would like to have the ability to load in agents (more-or-less vehicles) dynamically at runtime into the simulation. The goal is to be able to have agents developed completely outside of delphyne, and then loaded into a scenario. The basic tasks here are:

Work Items

• Get the low-level plumbing working for agents, probably using the ignition-common plugin subsystem Loadable agents core #213
• Define an abstract base class that all agent plugins will have to implement Loadable agents core #213
  • Base class needs to be a "System", in the drake sense of the word
  • Need to have inputs and outputs
  • Need to be able to parameterize the agent at startup time (pass in a map of parameters, maybe?)
• Conversion of existing Cars
  • Simple car: Add loadable prius simple car #237
  • Trajectory car Add in the LoadablePriusTrajectoryCar. #265
  • MOBIL car Add in a LoadableMobilControlledSimpleCar #301
  • Maliput car https://github.com/ToyotaResearchInstitute/delphyne/tree/add-loadable-maliput-railcar
• Integration with python https://github.com/ToyotaResearchInstitute/delphyne/tree/loadable-python
• Fix bug where return value from configure is not being checked
• [-] Formalize error handling from configure and from AddLoadableModule (skip till we have a need)
• Add default directory for plugin lookup: ~/.delphyne/plugins, with override from environment variable
• [-] Add further plugin override via YAML config file (currently uses an environment variable)
• [-] Add tests for the plugins (ala #300) - Probably don't need this if we switch to a has-a implementation, as the agent code and tests for them will be inside drake.

Documentation

• For users and developers in the Delphyne Guide
• Document configure virtual method to describe design and usage considerations Loadable agents core #213

Polish (see #346)

• Try out SHARED linking to drake and co. make sure it works for us
• Document in the delphyne guide somewhere our design choices ... and not STATIC and why!
• Open the question in ignition_common re RTLD_LAZY or RTLD_NOW.
• Refactor car references in automotive_simulator.cc to agents.
• Keep agent libraries separate (easy to know what's available on the filesystem) but collapse CMake code
• Bugfix segfault of the python script, Loadable agents script is segfaulting (MOBIL) #376
• (Daniel) Switch to a has-a rather than a is-a relationship and pull the drake car code itself
• (stretch)(Daniel) Deal with the setPlugin constraint
• (Daniel) Understand why the maliput car has delayed configuration (and causing us a second step in configuration) - This is because we can't set parameters until we have a context, and we don't have a context until we create a simulation object. The simulation object must be created after the system diagram is hooked together (i.e. Configure time), so I think we have to have a two-step setup here.

Integration

• Remove non-loadable code from automotive_simulator.cc (this may be somewhat difficult because most of the tests need to use AddPriusSimpleCar, so we'd have to figure out how to deal with that)

Demo Goal

• A new demo (not demo_launcher.py) enabled with the python layer that only loads and covers the loadable agents we have right now

Out of Scope

Dynamic reconfigure for scriptlet control will be left to a later milestone. Since it's a plugin framework with a fixed api, you can't just singly expose an api from this car, and yet another from another car. Likely some api with a generic key/value signature should help alleviate this situation.

Due to a limitation in ignition-common currently, implementations of the loadable vehicles have to derive from classes like: AgentPluginDoubleBase. In the future, it will be possible to templatize these, so the implementations can derive from AgentPluginBase<T>.

[stonier]

@clalancette I have a few questions with regards to the loadable agents. Let's catch up this week and summarise back here.

[caguero]

Shared linking to Drake libraries (#365).

[caguero]

Discussion about RTLD_LAZY vs RTLD_NOW in Ignition Common.
https://bitbucket.org/ignitionrobotics/ign-common/issues/34

[caguero]

Remove non-loadable code from automotive_simulator.cc:

• Remove non-loadable code from automotive_simulator #367
• Remove non-loadable code from automotive_simulator (part 2) #370

[caguero]

AddLoadableCar() has been renamed to AddLoadableAgent() in #371.

[stonier]

Looking good. Remaining work transferred to the final polish meta for M2: #378