Source height with multiple heat sources is ill-defined

[stvdwtt]

A few parts of the code need the current height of the build (particularly for adaptive meshing). As of #96, we base that on the scan path. However, if there are multiple heat sources and therefore multiple scan paths, the current height of the heat source is ill-defined. As of #96 we resolve this by taking the max height of the heat sources. However, this could cause undesired behavior if different heat sources are at different heights.

Especially if AMR is used, multiple heat sources should currently be used on the same level of the build to avoid unexpected behavior.