add checks on valid_on to read methods

[HomoCodens]

valid_on must be a vector of length 1 that is either a Date or an yyyy-mm-dd string.

How would we best/most elegantly enforce this?

[HomoCodens]

Actually, it may now have to be a Date at least when it is passed to dbQuoteLiteral. See #250.

Currently it is documented thusly:

#' @param valid_on \strong{character} representation of a date in the form of 'YYYY-MM-DD'. valid_on selects the
#' version of a time series that is valid at the specified time.