[Suggestion] Make it easier to add "date-only" values

[ehennestad]

Some openMINDS property value are date values with no time. A statement like affiliation.endDate = datetime("now") would currently not work because the property validator checks / asserts that the Time/Minute/Second values are 0.

Either create utility classes like date or be less strict about setting the value. An alternative is to allow datetime values but to silently remove the time information, either in a property set method, or when serialising values.