Small updates to Python collect and test routine

Making code more robust to missing data.
NetCDF C API code seems to work

Passes all basic tests, but still need to run drift and interchange
tests (laptop too weedy...)
Modified configure script to look for NetCDF C API

This API has remained quite stable across multiple versions
of NetCDF, whereas the C++ API has been changed significantly.
This code should therefore work on more systems.

The API used can be specified using an additional flag

--with-netcdfapi=c    Use the new C API (the default)
--with-netcdfapi=cxx  Use the older C++ API