Migrate Python scripts to modules and install via entry points

[tkittel]

For two reasons we should migrate Python scripts to modules and install via entry points.

1. To allow systematic access to all cmdline scripts from the python API, without messing with subprocesses.
2. To make it easier to install cmdline scripts on Windows, where pip can take care of all the .bat/.exe wrapping for us.

Once done properly, the python modules could be called like _cli_ncmat2cpp.py for ncrystal_ncmat2cpp, however they could in general come with a Python API frontend e.g. ncmat2cpp.py, where most of the features would be implemented. Or we remove the _ in _cli_ncmat2cpp in case someone feels like providing a custom arg list and call it exactly like the cli script.

Special cases: nctool (just the naming is special) and ncrystal-config (we might want this also as part of a future C++/C API package).

Relates to #154 and #152.