Read concatenated files

[jamespanton93]

Pull Request Checklist. Please read and check each box with an X. Delete any part not applicable.

Added capability to read a single netcdf file that is a concatenation of netcdf files of a single time step dump. Such files are create using ncecat which is available though netCDF Operators (NCO). This is useful for users who locally store lots of model output, where having a single concatenated netCDF file for each dump is more manageable than having a file for each process for each dump.

For all pull requests:

• [X] I have run the contrib/utilities/indent script from the main directory to indent my code.

For new features/models or changes of existing features:

• [X] I have tested my new feature locally to ensure it is correct.