Increase precision from CornerpointChopper::writeGrdecl

This change-set increases the precision of floating-point vector output from method `writeGrdecl()` of class `CornerpointChopper`.  Previously, we used the default precision (typically five or six decimal places) in the on-disk representation of the model samples generated by method `chop()`.  This, however, produced noticeable discrepancies in the results between models that were upscaled directly using one of the `upscale_*` utilities in module opm-upscaling and models that were chopped, written to disk and then upscaled in a separate pass.  Using more precision in the output vectors attenuates that round-off error effect.