Add argument to parallelized scripts to control number of cores used

[macovskym]

New argument --cores added to the following scripts:

• reconstruction/run.py
• random_networks/create_random_networks.py
• random_networks/compute_network_stats.py

[bruceamm]

--cores argument allows user to specify number of cores used, increased number of cores significantly speeds up computing time (no errors or issues, AB/IS)