Separately specifying the order for the heat source integration

[stvdwtt]

If possible, it could be useful to be able to specify a different element order just for the heat source. Often the heat source will be strongly localized and a low element order used for the thermal solve may not integrate it accurately. Accurate integration of the heat source is necessary to ensure the correct heat input.

This ability may be particularly useful in conjunction with the integrated heat source in #192, to calculate the long-range thermal thermal evolution on the part scale, where the part is much, much larger than the melt pool.