unit system: add temperature

this requires the possibility of specifying an offset for the SI
conversion because Eclipse in its eternal wisdom chooses to specify
temperatures using degrees Celsius and degrees Fahrenheit instead of
using Kelvin an Rankine...
add the keywords required to specify the initial reservoir temperature

E100 does not know about TEMPI, but it is wise not to use the
geothermal gradient directly, but to go over the "temperature at
cell-center" detour as specified by the E300 TEMPI keyword. The depth
vs. temperature table is given by the RTEMPVD keyword. The intend is
that if TEMPI stays unspecified the temperature is calculated at each
cell center and the result is put into TEMPI from where it gets picked
up by the simulator.

(Note that E300 says that TEMPVD is an alias for RTEMPVD even though
the possible sections of these two do not exhibit any overlap, WTF?)
add a table for RTEMPVD
add a grid property for TEMPI

by default it is initialized using the temperature vs depth table...
Merge pull request #370 from andlaus/basic_temperature_support

Basic temperature support