CreateMDHistoWorkspace used a method in ImportMDHistoWorkspaceBase.cpp
to declare some of its properties. Decoupling these two classss
allows us to implement mandatory property changes to CreateM
CreateMDHistoWorkspace without affecting ImportMDHistoWorkspaceBase.cpp

Refs #13912

The algorithm documentation for this algorithm is actually up-to-date
correctly pointing out that the file stores the variance. The fix
brings the implementation in-line.

Refs #11209

The converted text is stored in separate .rst files in the docs directory.
Refs #9562

Refs #9523

but some small bits and pieces can remain.