Finish unit tests and regression tests

[tmcclintock]

Write unit tests for all modules:

• [X] averaging
• [X] bias
• [X] boostfactors
• [X] deltasigma
• [X] density
• [X] mass function
• [X] miscentering
• [X] xi

Write regression tests for all modules:

• [ ] averaging
• [ ] bias
• [ ] boostfactors
• [ ] deltasigma
• [ ] density
• [ ] mass function
• [ ] miscentering
• [ ] xi