WIP: Netcdf mixin

Will close #1007 (closed).

Add the option to import and export timeseries data to and from NetCDF.

There are several todos / things to look at in 2019:

• Importing rtc-tools parameters from a NetCDF file does not really make sense. It would be a good idea to add support for using several of the available data formats at the same time, to allow for example, timeseries in an nc-file and parameters in a csv-file.
• The current code does not yet support ensembles, I've already gathered two example nc-files with ensembles exported by FEWS. @vreeken, updating this merge request to also support ensembles might be a good first issue.
• The NetCDFMixin currently uses an optional rtcDataConfig and constructs a default variable name when no data config is available. Should be discussed whether we want to keep this this way, and if so PIMixin should do the same (some code might also be moved to the data config class in this case).
• The unit tests need to be expanded. See to the todo in test_netcdf.py. Also, the netCDF4 package is not yet available for the test-suite so all added tests are failing.

This branch should not be merged before merging the io-refactoring branch!