Windows build

[jhale]

Uses vcpkg and currently the split build for debugging. Remaining error related to finding _basixcpp library on Python import. Needs tidying (passing lots of options/environment variables around at the moment). Also would be good to try combined build.

[jhale]

Summary:

• No hard coded paths in our source.
• Combined install works without adhoc modifications to $PATH or Python's dll loader's internal concept of the PATH. All dlls (basix, openblas etc). end up in the same directory as the .pyd wrapper. Python's dll loader will search . for all runtime links, and it just works.
• Split install. vcpkg-provided dlls are put with the C++ library. The Python wrapper sits on its own. C++ runtime linking only succeeds if the C++ libraries are put on the PATH (totally standard). Python runtime linking only succeeds if and the Python DLL loader is told about the C++ runtime directory, which is consistent with the documentation (https://docs.python.org/3/library/os.html#os.add_dll_directory). This is currently done through a conftest.py file to execute the tests.
• No changes for Unix targets.

The nasty IFs can be removed once RedHat has a newer CMake (>3.21).

This could be tried in conda now @minrk.

[jhale]

I've merged this with the idea that it will certainly need another iteration once tested in Conda.