Create set_intervals() and assert_intervals() functions

[billdenney]

This pair of functions will set the intervals for a PKNCAdata object and verify that the intervals are valid, given the object.

set_intervals <- function(data, intervals) {} will take in two objects, the PKNCAdata object and the proposed intervals. It will then check that the intervals are valid, given the data object. If the intervals are valid, it will set them in the object. It will return the data object with the intervals set.

assert_intervals <- function(intervals, data) {} will verify that an interval definition is valid for a PKNCAdata object. Valid means that intervals are a data.frame (or data.frame-like object), that the column names are either the groupings of the PKNCAconc part of the PKNCAdata object or that they are one of the NCA parameters allowed (i.e. names(get.interval.cols())). It will return the intervals argument unchanged, or it will raise an error.

set_intervals() will be called to set the intervals within PKNCAdata(); assert_intervals() should be called both within set_intervals() and in assert_PKNCAdata() (see #328).

[billdenney]

Be sure that you can handle both grouped and ungrouped conc data. For example:

library(PKNCA)

o_conc <- PKNCAconc(as.data.frame(datasets::Theoph), conc~Time|Subject)
names(getGroups(o_conc))

o_conc <- PKNCAconc(as.data.frame(datasets::Theoph)[datasets::Theoph$Subject == 1,], conc~Time)
names(getGroups(o_conc))

[billdenney]

Completed in #335