Feature request: SDTM input functions

[Gero1999]

So in aNCA we are highly interested in simplifying more and more the mapping. I though that it might make sense to also have these functions available for PKNCA. The idea is to directly accept SDTM data (PC, EX) using their column assumptions and standard naming for the default arguments. That way if an user has a standard SDTM they can directly do:

o_dose <- ex_to_pkncadose(ex)
o_conc <- pc_to_pkncaconc(pc, ex)

or even

o_data <- sdtm_to_pkncadata(pc, ex, dm)

@billdenney what do you think? I will open a draft PR (#517) for you to check an example (ex_to_pkncadose), but feel free to close this issue/PR if is not seen as fitted for PKNCA