Open dataset in context manager

[ColCarroll]

This fixes #202 on my machine. I will check travis logs to see if it fixes it there, too.

[aseyboldt]

Do we actually want to close this file here? This means that we need to load everything into memory, right? For some model that might not even fit. Maybe it would be better if we behave a bit more like xarray itself here, and keep the file open as long as the inference data object exists. It could also be a context manager, so that people can make sure the file gets closed.

[aseyboldt]

It also looks to me as if we stumbled into a netcdf or xarray bug in #202. If we have a way to actually reproduce that with xarray alone, we should report that. I'm still failing to reproduce it myself though... :-)

[ahartikainen]

I think we can load data to memory at least for the first release.

And later if we can find out how to do it without killing python, implement it. E.g. use kw to define if data is loaded or not.