[new program] plot_dn_dM

- this program plots number of stars as a function of
  -mass
  -magnitude

- created to check if there was agreement with Figure 3
  in geha et al 2013 - there is. originally i thought that
  dn/dM looked different because I was comparing my Kroupa
  LF plot to a pure single power-law from that Figure 3.

- this program was adapted from code in simulate_cmd(...)
  and main.py