CMake: Add option to download and build netCDF C++ API

[ZedThree]

Fixes #2105

Currently uses my fork -- I've opened a PR, but it might be a while before it gets merged.

@johnomotani Any chance you could check this works on MARCONI?

[ZedThree]

John found he had to use -DCMAKE_PREFIX_PATH=$(nc-config --prefix) to get netcdf-cxx4 to find netcdf-c. I'll make a note of this in the docs. We actually had a bunch of doc changes go into master that aren't in next, so I need to do that first!

[johnomotani]

Is it possible to automate this? ie get cmake to pass $(nc-config --prefix) to the netcdf-cxx4 build by default? As I understand nc-config is a standard utility?

[ZedThree]

Yes, I think I just need to split the current FindNetCDF.cmake into two parts, one to find the C library, and one for the C++ API. That way, I can find the C library before downloading the C++ API, which should then let it skip its checks. I might then contribute the C version back too.

[johnomotani]

This works for me, configuring with just cmake -DBOUT_DOWNLOAD_NETCDF_CXX4=ON .. and tests pass. I wondered briefly whether it'd be good to add the -DBOUT_DOWNLOAD_NETCDF_CXX4=ON flag in a test, but we only have one run using cmake at the moment so I don't think it's worth it.

[ZedThree]

My changes had broken things for the older version of netCDF-C on Travis, should be fixed now

[ZedThree]

Also, I should note that the CMake module names have changed a little bit to be more in line with the upstream netCDF names, which are netCDF and netCDFCxx. This affects things like finding netcdf via netCDF_ROOT/netCDFCxx_ROOT CMake variables

[johnomotani]

auto-downloading netcdf-cxx4 still working on Marconi 👍