New program: simulate_cmd.py

- Start work on code to simulate CMD. The following steps
  have been implemented so far:

1. Input arguments:
   nstars : number of stars in MC CMD.
   age, feh, afe: parameters for isochrone to be called
   within program.
   dmod : distance modulus for simulated CMD.
   magarr, magerrarr: error as a function of magnitude,
     this is a way to specify errors to be used with the
     simulated data, though i may try with others later,
     e.g.
   system : either 'wfpc2' or 'sdss', currently only
   'wfpc2' is being updated.
   *kwargs - see below.

2. select salpeter or chabrier based on input *kwargs and
   exit program with error message if neither salpeter or
   chabrier option has been selected. if salpeter, require
   kwarg specifying alpha, if chabrier, require mc and
   sigmac (mc = mass in msun units, NOT in log, converted
   to log within f_chabrier).

3. select minimum and maximum masses, either as inputs to
   *kwargs, or setting them to default values within
   simulate_cmd.py

4. Generate a large number of pairs in Mass, dN/dM space
   and choose the first N, where N is the number of stars
   in the cmd to simulate. [to do: if less than N of the
   pairs were within the desired region, draw another set
   of pairs, until at least N pairs are within Mass,dN/dM
   envelope].