AUC estimation streamline conc scale for estimation - add argument

[sedavid01]

Referring back to previous conversations with SME's that utilize AUC for their summary statistics of dose-response models, AUC values for all models should be estimated on the same concentration scale.

Goal: Add functionality to AUC calculation functions that allows a user to determine whether the AUC should be estimated with the concentration on the raw or log-scale, and will estimate AUC's on a consistent scale with consistent interpretation across fits.

Needs:

• Add a new argument to the AUC functions allowing indication of log-scale concentration used for estimating the AUC value.
• Add functions without a log-scale re-parameterization that will be needed for the consistent AUC estimation on the log-scale.
• Update internal logic for get_AUC to accommodate for the new argument/logic flow of the function.
• Update documentation for the respective R functions.

Nice to have's:

• Message or warning regarding which scale the AUC is estimated in.
• Optional output providing "meta-data" on which concentration-scale is used for AUC estimation.