Remove old legacy unit constants.

They should have been removed a long time ago, but were overlooked.
Merge pull request #656 from atgeirr/cleanup-legacy-units

Remove old legacy unit constants.
EclipseWriter: clarify a comment
EclipseWriter: don't convert the unit inside the rate() function

because if that is done, the integration for the production total is
wrong because the time step length is days and not seconds.

Note that the conversion now uses Opm::unit::convert::to(<>,
1/conversionFactor) instead of simply multiplying with the conversion
factor. I still think this obfuscates things more than it helps, but
[at] bska will hopefully want it this way...
Merge pull request #654 from andlaus/fix_rate_conversion

Fix rate conversion