demographics.py missing an important functionality

[jdebacker]

We need to allow for demographics.py to be able to return path of population distributions when given (1) an initial distribution (2) paths of fertility, mortality, and immigration rates.

This could probably be best implemented in the demographics.get_pop() function with something like:

def get_pop(
    E=20,
    S=80,
    min_age=0,
    max_age=99,
    infer_pop=False
    fert_rates=None,
    mort_rates=None,
    infmort_rates=None,
    imm_rates=None,
    pre_pop_dist=None,
    country_id=UN_COUNTRY_CODE,
    start_year=START_YEAR,
    end_year=END_YEAR,
):
   if not infer_path:
       do current functionality

   else:
       find implied pop path from fert, mort, imm rates + either(pre_pop or UN data if pre pop not given)
    return pop_2D, pre_pop

[rickecon]

This is implemented by PR #902. Closing.