remove the "MATRIX_COMPRESSIBILITY" from ROCK

This field is specific to the Eclipse 300 geomechanics module. IMHO,
either all fields for the eclipse 300 geomechanics module should be
added or none. Adapting the table code to "partially defined" keywords
is a pain I'd rather avoid!
Merge pull request #116 from andlaus/remove_r300_column_in_rock

remove the "MATRIX_COMPRESSIBILITY" from ROCK