Merging Accumulated Bug Fixes

[zackphillips]

Sorry for the large PR - this fixes a few minor and somewhat major bugs, mostly in the AST parsing.

List of changes:

• Caches modules for faster creating (e.g. when creating many copies of the same module, pykicad only parses the .kicad_mod file once)
• Fixes a AST/pyparsing bug which caused an error when files had multiple user via sizes or user trace widths. Currently, pykicad just loads the first one, since supporting multiple keys of the same name seems to break the AST. This should probably be fixed eventually but isn't a huge deal in the near-term.
• Fixed a bug related to Kicad-side changes in CAD model objects
• Added and updated the .gitignore
• Fixed the unit tests. These should now pass by running pytest in the ./tests directory.
• Minor comments to files
• Minor changes in AST format to accommodate recent changes in Kicad file structure