Fix typos, and make Unions report more uncombined sets

[dpvc]

This fixes a couple of typos in AnswerCheckers.pm, makes sure that the derivative of a fraction copies the denominator when squaring it (otherwise the derivative shares a tree node with the original), and fixes an error in isReduced() for Union objects where unions like [0,1) U {1} would be reported as reduced (rather than requiring it to be [0,1]).

The latter can be checked using

Context("Interval")->flags->set(
  reduceUnions => 0,
  reduceSets => 0,
);

($reduced,$reason) = Union("[0,1) U {1}")->isReduced();
TEXT("Reduced = $reduced",$BR,"Reason = $reason");

With the patch, this should report "Reduced = 0, Reason = uncombined sets", while without the patch, it should be "Reduced = 1, Reason = " with no reason.

[mgage]

Passes the tests.