add tests

[brownsarahm]

• set them up based on pytest
• coverage for at least the populations
• add them to CI

general reference on testing

[brownsarahm]

Make tests gradually:

• Check that it all runs
• Check that constructors set right parameter values
• Check that sampled data is correct

[brownsarahm]

this was partially complted in #14 still need to add more tests