Filename and structure improvements

[edbennett]

There are some Python conventions that will make the code more idiomatic to use if you can follow them.

• Python module and variable names usually use snake_case rather than PascalCase. (The latter is used for class names.) So things like InverseProblemWrapper.py -> inverse_problem_wrapper.py
• Is _class needed in filenames? I would imagine that from .gphlt import HLTWGPrapper` would be more fluent.
  • (Is that class name a typo? Should there not be a Wrapper in there?)
• Does it make sense to use subdirectories to add more structure? (You may already have done this as part of Use Python package structure #27)