Improve the XtcePacketDefinition API

[medley56]

Context

The packet definition object represents the XTCE document but not all attributes are currently exposed. Users should be able to access all of the packet definition information via the packet definition properties.

Implementation Plan

• Change named_parameters to parameter_set
• Change named_containers to sequence_container_set
• Improve the repr methods for Parameter, ParameterType, DataEncoding, and Calibrator so if the user accesses those properties, they are useful.

[greglucas]

Along a similar vein with working on this API: Is there a way to "know" the root container? Right now we have the user pass the root_container_name into the packet generator. I'm wondering if this means there can be multiple "roots"? Alternatively are we able to just detect that automatically for a user and start with the sequence container that is the root we found. Another option would be to have the root_container_name be passed to the XtcePacketDefinition constructor instead of the packet generator.