stellar relaxation should not repeat itself

[danieljprice]

The relax-o-matic procedure for setting up stars is now automated (since #411) to make many two-step setup-and-moddump procedures into one-step procedures.

However the disadvantage of this is that if one wishes to change e.g. orbital parameters in the .setup file one must run the stellar relaxation over and over again.

Ideally relax_star would detect if a relaxed star that matches the desired profile and resolution exists in the current directory, and use this instead of repeating the relaxation procedure.

Related to this, running phantomsetup, quitting half way through and running phantomsetup again should pick up where it left off even if it was part-way through the relaxation procedure.

Should not be too hard to implement...