Add custom build backend for parallel NetCDF4

[stubbiali]

Introducing an in-tree build backend which programmatically detects whether NetCDF4 supports parallel I/O, and if so adds MPI4Py to the list of build dependencies. The custom backend is a very thin wrapper around the original Setuptools backend.

[CLAassistant]

All committers have signed the CLA.

[ZedThree]

You might want to report which nc-config it's trying to run. Currently the Windows builds are failing with FileNotFoundError: [WinError 2] The system cannot find the file specified which doesn't include the path, annoyingly!