return precipitation bias adjustment factors to run_simulation remove 0.5<x<2 threshold

[btobers]

Addresses #118, such that glaciers with a precip bias adjustment factor greater than 2 no longer fail to run due to an assertion.

[btobers]

Reverted to assessing the gcm precip bias adjustment fraction as the factor of the bias corrected GCM precipitation to the reference period precipitation over the same time period. This check shows that the precipitation after bias adjustment should not differ greatly form the reference climate data.

E.g. line 417:

    gcm_prec_biasadj_frac = gcm_prec_biasadj_subset.sum(axis=1) / ref_prec.sum(axis=1)

Now in the case that the factor is not between 0.5 and 2, a warning will be raised in run_simulation.py, line 651, but the simulation will keep running.

@drounce would you mind giving this a review?