Add dense basis method

[christopherlovell]

Added the dense basis method as a parametric functional SFH component.

https://github.com/kartheikiyer/dense_basis

Works exactly like any other functional SFH form. Example plotting script provided. Added dense_basis module and statsmodels modules to setup.

Also added a plotting method to the common base SFH component class.

Waiting for update to dense basis code to remove superfluous dependencies before merging.

Issue Type

• Enhancement

Checklist

• [x] I have read the CONTRIBUTING.md -->
• [x] I have added docstrings to all methods
• [x] I have added sufficient comments to all lines
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no pep8 errors
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes