Replace normal with single-tailed distribution(s) for initializing non-negative values

[artgoldberg]

1. Support distributions other than normal: E.g., for initializing volume and species counts or concentration. Use a single-tailed distribution that does not have negative values, like Gamma.

2. Don't round() copy numbers in sample_copy_num_from_concentration, which just adds noise to the initial conditions. Rather, let the species_population module round copy numbers as needed for discrete-time integrators.

[artgoldberg]

2. is done