drop nans for datasets

[MerlinSmiles]

I just made a new issue to rephrase microsoft/Qcodes#208 to a feature request

When loading an experiment file, that was not written to the end, i.e. by aborting a measurement, the missing values from the original intended sizes are just filled with nan's.

It would be nice to have a dropna function like in pandas that removes all dataset rows at the end of the dataset which are filled with nans, maybe also all colums that are nans only:

DataSet.dropna(axis=0, how='all') #removes all columns with nans-only or setpoints-only
DataSet.dropna(axis=1, how='all') #removes all rows that have nans-only

including the inplace feature that updates the dataset file, and the snapshot arrays.

Even though this discusses the DataSet it should also be possible for a single DataArray :)