refs #7962. Provide documentation for functions.

Start cleanup. Add new method which wraps original combine2 method, so that workspaces rather than workspace keys can be dealt with.
refs #7962. Remove magic naming strings from combine2.

These magic strings make the code completely unreadable, so I'm refactoring them out.
refs #7962. Refactor combine2 method.

Is now more pythonic. Utilise MatrixWorkspace algebra where possible. Explicitly indicate that masking is occurring in the latter MultiplyRange calls. Add Documentation for key pieces.
refs #7962. Simplify the script as much as possible.
refs #7962. Do not modify input workspaces as part of this script.
Merge branch 'feature/7962_combine_multi' into develop