Downcast parameter data types

[jdebacker]

This PR attempts to address Issue #1080, but using more memory-efficient datatypes in the Specifications class object: 16-bit integers and 32-bit floating point numbers replace 64-bit integers and 64-bit floating point numbers.

[jdebacker]

Note that I have adjusted the default steady-state resource constraint tolerance. With the downcast parameters, I noticed a SS resource constraint error of -4.92456176e-08.

I'm not sure, but I'm wondering if this might have to do with the rounding of the demographics parameter which means that the omega may not sum to exactly 1 and there may be some (small) difference between the rho and imm_rates and the omega they imply. This could be inspected by using 32-bit floating point in the demographics.get_pop_objs functions. I have not done such inspection at this point and only ran the run_ogcore_example.py with the default parameters.