Generalize how we use quantities from previous files

[astrofrog]

In the same way as we have an option to use the specific energy from another file, we should be able to use a density and a minimum specific energy from a previous file. We could even envisage a 'continue' method that makes it easy to continue a previous model:

m.continue('other_model.rtout')

But it would need to be obvious how much is copied in this case. It might be easier to do:

m.copy_quantities('other_model.rtout', ['density', 'specific_energy'])

or similar.

[astrofrog]

Could then also have link_quantities, to make it obvious that one is a copy and the other is a link? Although maybe it would be best to have a more general method with a copy argument:

m.use_quantities('other_model.rtout', ['density'], copy=True)

Also, the default for the quantities could be 'all'.

[astrofrog]

Also add method to read in geometry:

m.use_geometry('other_model.rtout')

[astrofrog]

These are now implemented