attributes of hRIXS class

Currently, running:

h = hRIXS()
h.get_params()

returns an error AttributeError: 'hRIXS' object has no attribute 'PROPOSAL'

Also, if we do:

h1 = hRIXS()
h1.FIELDS += ['test']

h2 = hRIXS()

then h2.FIELDS also contains 'test', which doesn't seem to make much sense (although it makes sense from a Python point of view)

Why not solve all this by having a constructor taking a proposal number as parameter and define all attributes in it? I don't see a use case where we don't want to set the proposal at the moment of creation.

This is what is more or less what is done here for the Viking: 9d61955d. The aim is to handle the Viking class in a similar way as the hRIXS, so suggestions are welcome.

@teichman @kuikenb