Look for keyword 'ROCKTYPE' if 'SATNUM' is not found

Some eclipse grid files include keyword 'ROCKTYPE' instead of 'SATNUM'. This is for instance the case for some grids exported from SBED. 

When using the upscaling application `steadystate_test_implicit`, not checking for 'ROCKTYPE' can be problematic since the routine now assumes that only one stone type is present and the first stone file will be used for all cells. 

I'm not sure if this problem is present elsewhere in the source code. At least this resolved my issue when running `steadystate_test_implicit`. For the applications `upscale_relperm` and `upscale_relpermvisc` this  check is already implemented.