units: replace "t" with "Time"

this makes things more explicit and more readable
units: Replace "L" by "Length"
units: Replace "P" by "Pressure"
units replace: "mu" by "Viscosity"
units: rename "1/Rs to "OilDissolutionFactor" and "Rs to "GasDissolutionFactor"
units: rename "K" to "Permeability"
units: rename "Rho" to "Density"
units: rename "m" to "Mass"
units: introduce a "ContextDependent" dimension

this is meant for those nasty keywords where the dimension of an entry
depends on the user-defined value. One example are the surface rates
of the produced fluids for the ?CON(INJ|PROD).* keywords which is
different depending on whether the user choses to control for the
surface gas, oil or water rate.

the approach taken in this patch is to convert all numbers to NAN if
the unit is queried in SI (via item->getSIDouble()). Maybe it is
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 possible
that I missed some...