Disposition CSV-based packet definition code

[medley56]

There is some old CSV-based packet definition code that has not been worked on in a long time. This code works fundamentally differently from the XTCE code and the functionality is much more limited.

Specific issues that need attention:

1. Documentation of CSV format supported by the CsvPacketDefinition class.
2. Improved unit tests for CSV-based packet parsing.
3. Separation of concerns between CSV-based parsing and XtcePacketDefinition. There are currently deprecated methods in XtcePacketDefinition that are only there in order to prevent breakage of the CSV-based parser.

We can either take the action to make these updates or we can just remove the CSV parsing from the package. I'd rather do the former because I want to support projects that can't/won't generate XTCE.