Implemented Kroupa-IMF Mock Sample Code

1) [myanalysis.py], NEW module f_kroupa:
   - takes in two free parameters (other than mass_arr,
     mass_min and mass_max which are in common with
     f_salpeter and f_chabrier):
     - alpha_1 : (neg of) slope for 0.08 < M < 0.5 Msun
     - alpha_2 : (neg of) slope for 0.50 < M < 1.0 Msun
   - slope for M > 1.0 sun set to alpha_3 = 2.3, not free
     parameter but can be changed later.
   - Function scaled in order to be continuous at the
     change in slope (see lines defining f_23 and f_12)
   - Mass ranges for each slope are hardwired as 0.08 Msun,
     0.50 Msun and appear in various places in f_kroupa.
     IF changed later, remember to change values througout
     function: e.g. in f_23 and f_12, dN_arr = ..., etc.

2) [myanalysis.py], updated simulate_cmd:
   - Added option to take-in kroupa parameters alpha_1 and
     alpha_2 into **kwargs (also need **imftype = 'kroupa')
   - Defined yum array (if testing == 1) for **imftype = 'kroupa'
     case.

3) [main.py], within sim == 1 if statement:
  - tested that choosing imfype = 'kroupa' works, by specifying
    alpha_in_1 = 1.00, alpha_in_2 = 3.00 (arbitrarily chosen).
  - added imftype = 'kroupa' to the if statement within which
    phot = simulate_cmd(...) is called.

4) [main.py], within sim == 1 if statement:
  - filter_phot: added x1=-1.5, x2=2.5, y1=24.3, y2=y2max optional **kwargs
    just as in main_eda.py. Also added option to lower mass_min_global to
    0.12 Msun if y2 > 29.4. These tweaks were implemented in main_eda.py
    and simply not updated here. This way I can change the color/mag bounds
    used for fitting of HST simulated data.
  - replaced fixed "28.5" for y2max in call to estimate_required_n(....)

5) [myanalysis.py], within estimate_required_n(...):
  - added if option for imftype = 'kroupa', calling f_kroupa, etc...