Consider generating XDL Builtin Types from a script similar to #22

[ahuston-0]

Currently util/datatype.hh and util/datatype.cc have to be updated by hand whenever we add a new builtin XDL Type, and while this isn't the worst thing in the world, we should really consider just auto-generating the class and header so we can just add a line to a JSON file whenever we update std.cc to have a new type.

Proposal:

• [x] Create a file /proj/xdl_types.json
  • [x] Insert each type in a format similar to "TYPE": "Type String"
• [x] Create a script /proj/xdl_type_gen.py
  • [x] Read in each key-value pair
  • [x] Write out all the keys (Types) to datatype.hh as part of the existing enum
  • [x] Write out all the values (Type Strings) to datatype.cc as part of the existing enum
• [x] Add execution of /proj/xdl_type_gen.py to CMake
• [x] Make sure build waits for /proj/error_gen.py to run and has all required types

[ahuston-0]

Note: There is a bit more code to write for this one than in #22 because there are other libraries and functions that need to get generated, but those should not change and can probably be written out as a constant string.