Unit tests for popsyn module

[elizabethteng]

Priorities:

• test_io.py
• test_rate_calculation.py
• test_synthetic_population.py
  • PopulationRunner
  • DFInterface
  • History
  • Oneline
  • PopulationIO (changed by Max PR)
  • Population (reworked by Max PR)
  • TransientPopulation (reworked by Max PR)
  • Rates
• test_transient_select_funcs.py

Others:

• test_analysis.py
• test_binarypopulation.py
• test_defaults.py
• test_GRB.py
• test_independent_sample.py fix name of binary_fraction_value to generate_binary_fraction
• test_sample_from_file.py
• test_selection_effects.py
• test_star_formation_history.py

[maxbriel]

I've excluded get_formation_times in star_formation_history in PR #543 but this function should be tested :)

[elizabethteng]

successfully fixed rebase, now I need to go back and copy over changes from v2.2 from Max's pops PR