allow unit specification when reading sink particle creation options

[danieljprice]

for the sink particle creation there are several parameters like rcrit, hacc, rsoft, rmerge that are often desirable to set to round numbers in physical units (like 1 au or 500 au). It would be helpful to be able to read these parameters as strings-with-units similar to how variables are handled in the .setup file using the in_code_units() functionality,

e.g.

     h_soft_sinksink =       0.1 au    ! softening length between sink particles