Feature: reorg inversion_solution modules

[chrisbc]

The current inheritance model used in this package has become difficult to handle and difficult to document well.

Suggested improvements:

• make *File classes (IS and FSS) members rather than supertypes of the user class.
• hoist *File methods to the IS and FSS only where necessary.
• make *Operations classes (IS and FSS) members rather than supertypes of the user class.
• hoist *Operations methods to the IS and FSS only if necessary..but consider not doing this but forcing users to migrate to new DF member.
• rename ...Operations classes to Model
• refactor the type *Protocol module splitting out *File and *Operations into their own classes
• migrate docstrings into the Protocol classes
• reorganise tests to align with the new module features