Generalise NeXus importer

[domna]

Currently, the nexus import can only deal with simple psi/delta files and ignores additional measurement parameters and times. This may be generalised in two fields:

• Additional fields such as the free parameter and time should be read out from the nxs file as well. Here it is maybe useful to use xarray instead of pandas. This would require an update of the decorators - but it is also possible to merge the simple psi/delta decorator into a multi fit decorator to just deal with xarray structures.
• The labels should just be taken from the appropriate fields in the nxs, such that the resulting structure represents the underlying file. This requires a change in the decorators, too (see #53).