Accessibility

This is the PG pull for the Accessiblity WeBWorK pull request.  Changes in this pull include
-  Added href to knows link so solution and hint knows should be tabbable
-  Added aria-labels to the inputs generated by PG.  They aren't real labels so things like Wave still complain, but they should be good enough.
  -  Test by looking for aria labels on answers.  They should be "answer x".  
  -  If there is only one answer then it should be labeled "answer"
  -  If there is a matrix it should be labeled "answer x row n column m"
  -  If the answer is on a GatewayQuiz it should be labeled "problem x answer y" 
  -  If the answer is part of a multianswer with singleResult=>1 should be labeled "answer x part y"
  -  Putting matrix or multianswer problems in a gateway quiz should do the right thing.  
-  Added BLABEL and ELABEL to PGbasicmacros.  If you do something like 
```
$BLABEL \(f(x)=\) \{ANS_RULE()\} $ELABEL
```
It should produce  a properly formatted input.