problem setting up a disc with binary potential

[martinatoscani]

when setting up a fixed binary + disc there is a problem in setting m1 and m2: massratio (binarymassr) is ambiguous and confusing in the code. Primary and secondary mass definition is not in line with what expected theoretically, which causes problem in the definition of the potential. Disc is behaving in weird way if if unequal binary mass objects are used.

Proposed solution: remove the parameter binarymassr in extern_binary.f90 and replace with m1 and m2 similarly to what is in extern_binary_gw.f90 .

We also need to change this in the setup routine for consistency.