Add ty type checker and fix errors

[elbersb]

• Adds ty as the default type checker
• Fixes ~300 errors that ty identifies
• Removes unused functions (dead code)
  • bayesian_base.py: _all_groups - unused method
  • bayesian_base.py: as_cumulative - unused (and broken) classmethod
  • confidence_utils.py: applyParallel - an unused parallel apply function for DataFrames
  • sample_size_computer.py: compute_summary - unused (and broken) method for computing summary statistics
  • constants.py: Z_CRIT, CORRECTION_METHODS_THAT_DONT_REQUIRE_METRIC_INFO