make the `Project` class pickleable

[alexhroom]

It would be useful to pickle the Project class, in particular for use of a ProcessPoolExecutor for asynchronously running RAT. However, this is tricky:

• when attempting to use pickle, the ClassList class is not pickleable
  • this can be solved using the dill library which provides pickling for a wider range of objects
• however with both pickle and dill, the input types for RAT_main are not pickleable. In general, it seems that pybind-generated classes cannot be pickled.

To pickle Project, we need to find a way of pickling the input types for RAT_main and then the whole class can be pickled via dill.

[alexhroom]

new possible method: just write a pickle function which converts to json and pickles, & the reverses for unpickling