allow fn!=hydromod to return sim (same as hydromod)

[jthurner]

Allows custom fn functions to return the model results along with GoF, in the same way as hydromod.

For the parallelised branches, I only tested "parallel" on Linux (although multicore is long deprecated and "parallelWIN" should return identical to "parallel").

[jthurner]

Rebased on top of current master.