Attempt to fix issue with trailCrossings

[bacchanalia]

This is an attempted fix for issue #337. While it gets rid of some
instances where test falsely returns a zero value, it is possible it may
introduce some instances where test falsely returns a non-zero value.

[bacchanalia]

Some outstanding questions I have relating to this PR:

1. Are there any cases where 1 < t < 1+eps where we actually want to signFromDerivAt to return 0 instead of 1?
2. Are there any cases where -eps < t < 0 where we actually want to signFromDerivAt to return 0 instead of -1?
3. Are the changes form 0 to -eps necessary or are they prevented because of the change in sign?
4. Are we worried about fp error in ang and do should we have eps terms in those comparisons also?