Improved parallelNelderMead (#300)

* Improved parallelNelderMead

Made some fixes to HARK.parallel.parallelNelderMead:

1) Renamed maxcores to maxthreads, to be more correct
2) Fixed various small comments
3) Applied fix if degree of parallelization is illegal
4) Fixed computation of x_dist (whoops)
5) Allowed initial simplex to be saved to disk
6) Adjusted save/load procedure to use comma as delimiter rather than space (and use one line per simplex point), to make it easier to copy-paste points from the simplex file while estimation is in progress.

Also deleted superfluous test of parallelNelderMead in the main block, and trimmed down the message that's printed if HARK.parallel is ever executed.

* use with open() instead of open/close for file handling

* fix linting changes

Co-authored-by: Christopher Llorracc Carroll <[email protected]>
Co-authored-by: Mridul Seth <[email protected]>