refs #7397. Move shared methods into base class.
refs #7397. Expose splitting.

Expose box splitting by making this algorithm a derived class of the BoxControllerSettingsAlgorithm. Also set sensible defaults for these splitting parameters. Fix the way that the workspace is created such that the box signal and error values are recalculated correctly.
refs #7397. Additional tests provided.
Merge branch 'feature/7397_reflectometry_md_splitting' into develop