set_ptrm().

allow assignment statements with mixed dlib::matrix and mat(T*) expressions
that might alias each other.  Also updated BLAS bindings to bind to set_ptrm()
assignments.

Setup py

if no cmake generator is set use same logic as cmake to find default buid generator but use Win64 if the build is for 64bit Python

avoid foreign artifacts potentially left over from VM builts.

make sure --cmake can be used to pass any cmake not in the path.