Refactor xtcedef.py into sub modules

[medley56]

The xtcedef.py module is about 2500 lines of code and the accompanying test module is about 1800. These are too big and make navigating the codebase annoying.

Refactor xtcedef and its test module to split up into different groups. Perhaps:

• common classes (e.g. ABCs)
• exceptions
• encodings
• parameter_types
• parameters

Could put the top level XtceDefinition class either in init.py or in its own submodule.