Formalise rules around create methods

#### Description:

The create methods needs some level of compliance. The initial state was that they:
- Need to sit in a file or folder matching the return type (this is fine)
- Need to have a method name matching the file name (this has proven not fine, and is being turned off in https://github.com/BHoM/Test_Toolkit/pull/529

Simply removing the separate check, without any additional guidance might be a bit to relaxed.

Could for example be that create methods must live in a file that is _either_ matching file name _or_ matching return type.

Happy for other suggestions though.