Add `.nc` and `.cdf` to the list of file extensions

[veita]

Currently the file open dialogs filter for .h5, .hdf4, .hdf, .h4, .he5, .he2, .hdf5.

It would be nice if .nc and .cdf could be added to support Network Common Data Form 4.

https://docs.unidata.ucar.edu/netcdf-c/current/faq.html#What-convention-should-be-used-for-the-names-of-netCDF-files

[byrnHDF]

Extensions can be added in the Tools/User Options menu and The HDF Settings page.