Add return line to simulate_cmd (test version)

- Previously code only generated dummy photometric data,
  i.e. magnitudes in two bands.

- Additions:
  - assign errors to each point based on their magnitude (their
    magnitude before adding errors, based on the mag-magerr
    input arrays)

  - create a np struct array with the same names as if it were real
    data, so it can be read in main.py.

  - currently, includes magnitudes, mag errors, 'color','colorerr',
    and 'covar', where 'covar' is set to 0.0 as in read_phot (may
    be modified later?) and 'colorerr' is also calculated as in
    read_phot().

  - [future: IF want to simulate spatial distribution, may want to
    add ra, dec to dummy array, also velocity, and so on...]