Improve defaults handling

this implements support for defaulting "intermediate" data points in items. One example are grid property keywords like

SWL
10*1 10* 10*2 /

another are partially defaulted PVT and saturation tables.

the aggregate of this patch set is quite large, but I tried hard to split it into meaningful chunks. (in fact, the splitting process took quite a bit longer than the actual development of the functionality, but I think it was worth it anyway because I found some bugs in the process, the stuff is easier to review and upon bisecting one does not have to find the regression in a thousand+ line patch. While we're at bisecting: The series is fully bisectable, i.e. after each patch everything builds and all unit tests pass...)