`_setDateTimePickerExent` takes four params. This could be improved (possibly) by adding a method to the picker class (e.g. `enableWithExtentAndValue()`, or something less wordy) and calling that directly.

Possibly even `enableWithLayer` which then leaves it up to the picker to pull things out of layer itself, although this is debatable because then the picker class has to know about `NcWMS`...