Fix MultiAnswer to set the correct_ans_latex_string

Fix MultiAnswer to set the corect_ans_latex_string, and also make it use the format and tex_format strings for the correct answer when singleResult is used.

This can be tested using

```
loadMacros("parserMultiAnswer.pl");
Context("Matrix");
$M = Compute("[[1/2,1],[2,3]]");
$r = Real(1);
$ma = MultiAnswer($M,1)->with(
  singleResult => 1,
  format => "%s and %s",
  tex_format => "%s \hbox{ and } %s",
  checker => sub {1},     # we don't care about the checking, only the correct answer display
);
BEGIN_TEXT
Matrix and number: \{$ma->ans_rule\} and \{$ma->ans_rule\}
END_TEXT
ANS($ma->cmp);
```

Check the `Show Correct Answers` and hit the `Check Answers` button.  Without the fix the correct answer will be `[[1/2,1],[2,3]]; 1`, with the fix, is should be a properly formatted matrix with the word " and " between the matrix and the number.