Add a "none" option in the calCurves argument in calibrate()

[ercrema]

Some dataset might contain dates not requiring any calibration (e.g. historical artefacts with known dates) - setting the calCurve argument to "none" should return a CalDates class object where the entire probability mass is in the calendar date provided by the user (i.e. the argument x). A warning message should be supplied in case the matching error is not 0 or NA.