Mod handles more simplification; geometry gets reflect method

This builds off of my mod branch (#1913) and does more simplifications of mod (without using nsimplify which is an approximation) and uses this in the reflect method that was added to geometry which allows geometric entities to be reflected across a line.

There are also changes to equals which allow it to prove that previously undecided expressions are or are not zero. This means that equals can be called from sign and abs to help them decide on some expressions that previously didn't evaluate.

An important upgrade to Factors was made; the deficiency was discovered while making the other changes above.