Allow users to pass a json file specifying the model

This MR allows users to specify models using a json file. This will make it possible to more easily pass arguments to source model classes.

An example json file is

{
  "module": "gwpopulation.models.mass",
  "class": "SinglePeakSmoothedMassDistribution",
  "kwargs": {
    "mmin": 5,
    "mmax": 50
  }
}