ConceptDatatype should support isTime() and isDatetime()

Currently, ConceptDatatype.isDate() returns true if the concept is a Date, Time, or Datetime.

We should create isTime() and isDatetime() methods so that we can test if a concept is specifically of type Time or Datetime.

We will have to figure out an elegant way of handling isDate(), because ideally we'd now want isDate() to return true only if the concept is of type Date, but we will need to maintain backwards compatibility.