use the same units for pressure output as for pressure input from the deck

before, the output was always metric, regardless of whether the deck
used metric or field units...
EclipseWriter: replace /= by unit::convert::to()

as insisted on by [at] bska
Merge pull request #651 from andlaus/EclipseWriter_use_deck_pressure_units

use the same units for pressure output as for pressure input from the deck