Findopm-parser.cmake: Fix it up, next try

thanks to Arne Morten Kvarving, Bård Skaflestad and Roland Kaufmann
for the discussion.
add the boost::regex library as a prerequisite

this is required for regex-matching keywords. Once GCC 4.9 is the
minimum compiler version to be supported, this can be dropped in favor
of std::regex ...
Merge pull request #162 from andlaus/sync_Findopm-parser.cmake

Sync findopm parser.cmake
Fix one-sided gravity potential calculation

Commit 5112b8a misinterpreted the role of index 'i' and, as a result,
installed code that would only define one-sided gravity potentials of
the first cell (roughly)--albeit eventually using the geometry of the
last active cell.

This commit restores the original, intended behaviour.