outputField(): Parametrise number of items per line

This is in preparation of leveraging the outputField() helper for
printing COORD and ZCORN too.
writeGrdecl(): Use outputField() for COORD and ZCORN

This leverages the "number-of-items-per-line" parameter introduced
in commit 608d4c1.
outputField(): Use straight-line code

There is no need to use a nested loop to output a constant number of
items per line.  We only need to output a '\n' rather than a ' '
after every "nl" item output.
Remove long-disabled code

The code to condense the output using repeat counts was disabled
long ago (commit fc992da).  If we ever need this feature, we can
bring it back from history or reimplemented in a refined fashion.
Merge pull request #431 from bska/simplify-cpchop-output

CornerPointChopper: Tidy code to output GRDECL format of subsample