Change to allow OpenMP to be enabled from command line before running a .ty script

To allow OpenMP to be enabled before running a .ty script from command line, code change was made such that "openmp=True" parameter is added to the command line.  This is to also ensure if some models may require the original default behaviour.  Change has been tested with passing parameter to the command line e.g. "topographica -p openmp=True tiny.ty" or "topographica -p openmp_threads=2 -p openmp=True tiny.ty".
Note that with this change, "openmp" parameter should be the last of all openmp related parameters to be defined in the command line.

Please advise if documentation needs to be updated accordingly and the best spot to do so.

Background: OpenMP was only enabled after running the .ty script as the default behaviour.