A checkFunction that checks if v has
one of the classes supported by dataMaid, namely character,
factor, numeric, integer, labelled,
logical and Date (inlcuding other classes that inherits
from any of these classes). A user supported list can be provided
in the treatXasY argument, which will let the user decide
how unsupported classes should be treated. This
function is intended for use as a precheck in makeDataReport.