multiple updates

- changed mass limits:
  - mass_min_global, mass_max_global are the ranges used
    to calculate the mock sample, and to estimate the
    required number of input stars in order to get the
    desired number in a more limited magnitude range.

  - mass_min_fit, mass_max_fit are the masses corresponding
    to the range of magnitudes used for fitting.

  - isochrones are no longer cut between mass_min_fit and mass_max_fit.
    in likelihood, isochrone points far away from a given data point
    will be weighed down by exp(-...). if isochrone is cut, it
    produces edge effects on points close to mass_min_fit/mass_max_fit
    as these data points *could* be at greater/smaller magnitudes.

  - temporarily, have replaced likelihood function by likelihood_nocovar
    function, since covar = 0.0 for now, this way it is easier to track
    down what is actually happening in likelihood calculation.

  - added a third subpanel to results plot: dN as a function of magnitude,
    mainly as a sanity check to make sure that simulate_cmd was creating
    samples with the right alpha parameter.

  - fixed legend in subpanels 1 and 2, now does show best-fit alpha value
    and alpha_in value.

  - best-fit alphas still far from input alpha.