Add as_refundObj.data.frame() function

[jeff-goldsmith]

Augment functionality in as_refundObj(), which currently supports only matrices, to allow the specification of variables that should be retained in the final data.frame.

For example

df = DTI[c("ID", "visit", "cca", "visit.time")]
as_refundObj(df, id = "ID", funcs = "cca")

should produce a data.frame with columns visit and visit.time in addition to id, index, value. This will be needed, I assume, for later function-on-scalar regressions and MFPCA.