ECL problem: implement rock compressibility

this makes eWoms match autodiff and Eclipse for SPE-1 if the injector
is disabled. with the injector it gets quite a bit closer, but it does
not yet match. (this is probably not a problem with the wells as
autodiff and eWoms agree that the maximum amount of gas should be
injected all the time and these rates are the same...)
FindERT.cmake: Look for libecl_well library