Merge pull request #447 from akva2/add_rho

added: nonstandard 'RHO' field in eclipse grid parser
changed: move the cpgrid chopper to a context based approach

needed to aid multithreading support
mark chopper methods const now that data is stored in a context
flipping the locale here is an issue with multithreads
add option to not rewrite registered factory entities when encountered

necessary with multithreading