Add support for different heat source orientations

[stvdwtt]

Currently we assume that the heat source is always directed downward. In many DED applications it's possible to rotate the build plate using the positioner to create more complex geometries and to e.g. deal with overhang geometries. If the positioner rotates out of plane, our downward facing heat source assumption is no longer valid.

I'm not sure what the best approach to dealing with this is.