'Full area' for bi-phasic or curves that cross the x-axis (get_AUC and AUC wrapper)

[sedavid01]

Goal: Add a logical argument to allow users to choose whether bi-phasic responses or curves that cross the x-axis have 'added' area or if they want to keep an 'as is' integral for the dose-response curve.

[gracezhihuizhao]

Only polynomial 2 has the potential to have bi-phasic shape that crosses x-axis, but how the codes are currently written, with positive constrain on its parameter "b", poly2 cannot be biphasic. Pushing this task back as of right now biphasic curve fitting is not valid.