units: introduce a "ContextDependent" dimension

this is meant for those nasty keywords where the dimension of an entry
depends on a user-defined value of some field of a (potentially
different) keyword. One example for this are the surface rates of the
produced fluids for the .CON(INJ|PROD).* keywords which exhibit
different units depending on whether the user choses to control for
the surface gas or the liquid rate.

the approach taken in this patch is to convert all numbers to NaN if
the unit is queried in SI (via item->getSIDouble()). It might be more
desireable to throw an exception in this case, but this approach would
be more elaborate and NaNs should be quickly noticeable by the users
of this code.

I tried to adapt all implemented keywords, but it's quite likely that
I missed some...