data_inspect() and select_data()

[clairemerker]

I added:

• data_inspect() to print the relevant content of the data. It takes an xarray.Dataset or a list of xarray.Datasets as input
• select_data() to select some specified variables. It takes an xarray.Dataset, a list of xarray.Datasets or a path to a file as input, and a list of variables. Optionally, one can also specify a grid file to combine.

One small remaining issue:

• If you specify a grid file, that grid file might not match all the specified variables if they are from different hypercubes. I don't know what the best way to deal with it would be, I currently exit with a more or less meaningful error message.