Started bug fix for arctan2 behaviour in prism

arctan2 not returns positive results for negative angles sometimes. The
fix corrects the angle depending on sign of input.
Added test for prism_gx around the prism

Need to make more for the other fields
Added test case for gy and gz around the prism
Created safe_log and safe_atan2 funcs for prism

Safe versions of atan2 and log that can handle the limits used in the
prism codes (log(0) and correct the sign of tangents).
Changed prism code back to 3 loops for integration
prism gxy fails below the prism at the corners

Weird behavior and it seems to happen with all cross components
Testing all components around the prism

The cross components all fail on the edges of the prism. Will add an if
for that case and add a dummy value to account for it.
New assertion for percentage in minunit
Move obs point if on corner of prism cross comps

The cross-components of the prism gravity gradient were failing on the
corners of the prism in one of the faces. Put a check if on that face,
then move the point some percentage of the prism dimensions.
Updated changelog. Made list of known issues
Merge branch 'master' into bug-prism-arctan2
Merge pull request #12 from leouieda/bug-prism-arctan2

Bug Fix: prism code giving strange results when below