changes
Fixed bug in ActionVolume

A virtual function that is referenced was named wrongly
Solve numerical issues in regtests

I added a script that converts "-0.0000" to " 0.0000" before
making the diff for regtests. This is done only in files
starting with "!# FIELDS "

To avoid false errors I also decreased the precision of a few files.

Fixes #29
Merge branch 'v2.1' into v2.1-travis-ci
Testing manual without MPI