Reads an outside-information kinship override table from a
user-supplied file into a data frame for checkKinshipOverrides
and reportGV. The expected long form is the output of
kinMatrix2LongForm: columns id1, id2, and
kinship, with a header row, so a user can export the current matrix,
edit a few rows, and feed it back. Excel (.xls/.xlsx) and
delimited text (.csv/.txt) files are both accepted, mirroring
getGenotypes.