AMR controller for unstructured meshes

A new type of AMR controller would be a good to have for unstructured meshes. Such a controller would incorporate the relative element size `h` measured in some way (e.g. smallest side length, aspect ratio, Jacobian, etc). Also a smallest `h` size should be specified similar to setting the `base_level` in the current controller. These `h` values would then be used during the computation of a `target_level` and assist in the decision of whether or not an element be refined / coarsened.

Start by reading the current AMR controller and the unstructured-mesh code paths that compute target_level and decide whether elements are refined or coarsened. Clarify how relative element size h should be measured and how a minimum h should correspond to base_level. Done means a working controller that uses h values for target-level and refinement/coarsening decisions.

Written by the indexing model from the issue text.