Implements BwdTrans StdMat operator

See merge request !2

Updated code to use a common base class and single factory pattern.

See merge request !1

Expand on the comments in main.cpp.

Explicit instantiation of OperatorFactory with doubles.

Tried to move boilerplate code up to base class.
Added operator type descriptor/trait structs.
Replaced enums with typenames.

Still has multiple definition linking errors to fix.