Adding a significant figure context

[sfiedle1]

Second attempt to submit a PR to Peter Stab's fork of the pg repository. The proposed macro will permit a context for significant figures.

[drgrice1]

@sfiedle1: This should not be a pull request to @pstaabp's pull request. This should be added as a pull request to the https://github.com/openwebwork/pg directly. It will need to be properly reviewed separately from this one.

[sfiedle1]

@drgrice1, @pstaabp: Thank you Glenn. This macro was originally intended to be included as joint work Peter and I were completing. We put a pin in it since there is another sig fig macro in development https://github.com/limefrogyank/WeBWorKChemistry with an enhanced feature set. However, I personally still use this perl-based sig fig macro in my problems and perhaps others in the community may also find it useful as well. Perhaps I may just post it in an appropriate thread on the WeBWorK Problems forum.