Update simulate_cmd after changes in read_phot

1) Parameters of desired synthetic data now passed to
   simulate_cmd() module include:
   system = 'acs','wfc3','wfpc2','sdss','cfht'
   age = 6,10,14
   feh = -2.5,-2.25,...,-.75
   afe = 0.0,0.4
   dmod = specify any value
   nstars = # of stars between mass_min and mass_max that go
    to form simulated CMD
   mass_min = idem
   mass_max = idem

2) simulate_cmd now takes magarr1,magerrarr1,magarr2,magerrarr2, two
   different relations between phot error as fnc of magnitude, one for
   each specified band