Load spatial means as an xarray dataset

Created originally on Bitbucket by avmo (Ashwin Vishnu)

Was already merged in Bitbucket before import, marked as merged by the import user

• new method load_dataset
• debug time_first_saved and time_last_saved . See https://stackoverflow.com/questions/48063908/unexpected-error-when-using-seek-function-in-a-file-python-3-6-3?noredirect=1
• sw1l solvers will use SpatialMeansJSON as base class with some deprecated backward compatibility, the json files are lighter in size and easier to read and extend.

I propose we use SpatialMeansJSON for other solvers too - but it will be a little bit of work.