simulate_cmd.py / esimate_required_n: fixed bugs

**simulate_cmd:
- read_iso no longer includes mass cuts, we read the entire isochrone,
  ie way magnitude cuts are done at the photometric level, not in mass.

- mass_min and mass_max are set AFTER reading-in isochrone. This way
  if the specified mass cuts fall below/above mass limits of isochrone,
  the mass limits are set to those of the isochrone.

- (around line 100): xrantmparr is now set between mass_min and mass_max,
  which are now definitely within the bounds of the isochrone [see point
  above].

** estimate_required_n:

- fixed return statement in estimate_required_n, is now of type int,
  otherwise when creating dummy arrays in simulate_cmd with nstars
  being float, program breaks.

-