See inline code comments for more details regarding the reasoning

    Avoids a bug in VS2017 where the call_once flag is not zero
    initialized but dynamically initialized despite its constructor
    being marked constexpr. This leaves the singleton
    initialization open to intialization ordering problems.

    A local static variables avoids the ordering problem and are
    guarateed to be threadsafe in C++ 11.

Refs #21800

Move the implementation of V3D operators to the header. This results in
a significant speed up in V3D operators.

The algorithm was setting the InOut *Workspace* property to another
workspace if the input workspace type was MatrixWorkspace. Instead, it
should modify the InOut value in place.

Re #23600

This will contain code commonly used by any part of the code
base that interacts with the Python C api.

Refs #23563