Windows installation fails checking for openmp support

[badpandabear]

Windows installation for PypeIt v 1.13 fails with a FileNotFoundError. This was reported in the users slack.

This is caused by the recent code added to pypeit/bspline/setup_packages.py that looks to see if openmp is supported. This code attempts to compile sample code, and fails on windows if it can't find the compiler.

A work around is to set the "CC" environment variable to a dummy command that will fail without an exception. In the users slack I suggested the windows "help" command, which will return a failure with no side effects. This allows the installation to succeed but does not allow openmp support to work.

To fix this I believe we should attempt to use the compiler mechanisms built into setuptools so that the test for openmp support uses the same compiler as will be used to build the bspline extension. Ideally we would also use the extension_helpers code to add the openmp compiler arguments, but if that fails I think we should support the Visual studio "/openmp" syntax as well as the gcc "-fopenmp" syntax.

The installation documentation should also be updated with instructions on what compiler to install on Windows prior to installing PypeIt.