Support 'SOIL' keyword

This commit introduces support for the SOIL (initial oil saturation)
keyword.  This is similar to existing keywords 'SWAT' and 'SGAS' and
needed to input some decks.
Merge pull request #220 from bska/support-SOIL

Support 'SOIL' keyword
Changed name of oil compressibility item.
Merge pull request #222 from joakim-hove/fix-PVCDO

Changed name of oil compressibility item.
Added Box() class to book keep the indices in a box.
Added constructor for sub box.