Add an output data object to FE::get_*_data().

Add an output data object to FE::get_*_data().

This allows finite elements to already fill certain output data elements
at the time the get_data() function is called, rather than having to later
figure out whether this is the first time (or not) they get into
fill_fe_*_values().

In its current form, the patch only adds that possibility to passing the
same output object to FiniteElement::get_*_data() that will later be used
when FiniteElement::fill_fe_*_values() will be called. The patch does not
actually implement any of the elements using this -- I will leave this
to a later patch addressing #1824.