Add viscosity to parameterfile
This MR adds the viscosity to the parameter file, removing it from being a compile-time option.
It goes quite deep into some of the hydro code, therefore:
- @jwillis, can you check your vectorised stuff is alright? I have had to make a minor modification and I think you can remove one of the values from the vector.
- @jkeger, can you fix up the balsara stuff in your planetary scheme? I've fixed it for the "with-balsara" case, but it might be best for you to fix it for the "without". This will also help you tidy up those ifdefs ;).
Pending my hydro test suite running smoothly, and those changes being made, this should be a relatively straightforward change.