Update answerHints to not produce hints in Preview mode

[dpvc]

This PR adjusts the answerHints macro to not produce answer hints in preview mode unless a flag is set allowing that.

See #468.

[taniwallach]

This seems to work as expected, including all the configuration/override switches, and that is a big improvement over the simple hack I had proposed in #468 .

---

I tested with the sample file from: #468 and then made some revisions to test with the switches.

As expected, by default - preview no longer shows the message.

Adding processPreview => 1 as below and a second message and third message (to test the various options):

   (-1 * $ans1) => [ "You seem to have a sign error", score =>0.50, processPreview => 1],
   (2 * $ans1) => [ "Your answer is double the correct answer", score =>0.25 ],
   (3 * $ans1) => [ "Your answer is double the correct answer", score =>0.10, processPreview => 0 ],

showed that processPreview overrode the new default behavior for a single message.

Adding answerHintsProcessPreview => 1 into the cmp() arguments changed the default, and quick testing was done also with answerHintsProcessPreview => 0.

---

Revised test file with use of switches:

DOCUMENT();
  
loadMacros(
   "PGstandard.pl",
   "MathObjects.pl",
   "answerHints.pl",
);

Context("Numeric");

$ans1 = Real(1);

Context()->texStrings;
BEGIN_TEXT
\( i^4 \) = \{ $ans1->ans_rule(3) \}
END_TEXT
Context()->normalStrings;

ANS( $ans1->cmp( answerHintsProcessPreview => 1 )->withPostFilter(AnswerHints(
   $ans1 => ["",replaceMessage=>0],
   (-1 * $ans1) => [ "You seem to have a sign error", score =>0.50, processPreview => 1],
   (2 * $ans1) => [ "Your answer is double the correct answer", score =>0.25 ],
   (3 * $ans1) => [ "Your answer is triple the correct answer", score =>0.10, processPreview => 0 ],

)));

ENDDOCUMENT();

[taniwallach]

Tested and working properly.

[drgrice1]

This works as expected. I am just not sure what is going on with lines 126-130.

[taniwallach]

Merging and closing #379