OPL problems should indicate a minimum required version of PG where applicable

[dlglin]

With new features being added to PG there should be a way to determine which OPL problems are compatible with your version of WeBWorK.

The easiest thing to implement would be to add a tag that indicated the minimum required version of PG. If we agree on a name for the tag then we can start adding it to new problems as they're added (and old problems that are rewritten), even if it's not implemented in the Library Browser yet.

Another question that may warrant a separate discussion: do we maintain backward compatible versions of problems if they are rewritten with new techniques? I'm loath to maintain multiple versions of problems, especially ones that have now been superseded, but this would facilitate people updating the OPL without updating their version of WW.