WIP: framework for gaussian heat source

[colemanjs]

Candidate branch for implementing gaussian heat source

[colemanjs]

@Rombur, just a note that this implementation is a work in progress for the definition of the heat source for HPC4. Based on the calibration results, this may change to a super-gaussian with a slightly different mathematical form. Once validation is complete, I can mark this ready for review

[Rombur]

Sounds good