69 poly2 biphasic modeling updates testing

[sedavid01]

Please review the developments, which include:

• Update of the poly2 from a monotonic only fitting to biphasic fitting allowance
• poly2 biphasic fitting allowance also required updates in the BMD estimation ONLY for the poly2 model

Additional notes:

• The updates were reviewed and tested on several data types that utilize tcplfit2 for dose-response model fitting, and results were similar across the board.
• Best fit results (particularly after hitcalling) are reported in the poly2 monotonic parameterization to maintain consistency across the output - should not be an issue since there is a direct conversion between two poly2 parameterizations

[sedavid01]

Note there do look to be issues with the unit tests, but many of these issues look to be data stream specific tests which is to be expected.

[sedavid01]

Updated documentation files per brief discussion with @madison-feshuk. Please let me know if there are further comments, questions, or concerns. Ready to be reviewed again.