Completed datetime_field_access

- Implemented forgotten cases justDate and justTime.
- Added tests + exceptions as in Rascal interpreter.

TODO: In both implementations UnsupportedOperation is thrown. I guess
this has to be replaced by something better, e.g.,
UnavailableInformation
Implemented datetime_updated.

Here again, UnsupportedOpetions exceptions are thrown that we should
review.

Added test cases.