Input files must have a file extension

[ccarels]

Description
Currently some files in the input_files/ have a .txt extension but some do not.

It would be good to have the code parse the file to look for a common extension.

My feeling is that the files currently under input files function more like macros, being different to actual input files which contain data to be processed.

Therefore, it might be worth considering renaming the input_files/ to macros/ and appending a .macro extension (or similar) instead of .txt, for the avoidance of doubt to the user.

Outcome
Uniform naming scheme for input files, clarification of their purpose to the user.
The code can parse and enforce the file extension.

Timeline
When convenient.

Interfaces
Will this feature result in a backwards-incompatible interface change: Yes. Existing input_files will need to be updated to include the appropriate file extension (as the code will no longer accept files without the correct file extension after this change).

Expected semantic version number increment category (Please indicate x.y.z): x.

Notes
Whatever the outcome of this issue, the interface for macros (currently called input files) must be documented.

[ccarels]

@KAdamek @jan2nov Thoughts?