fix introspection so that models without melt transport work again
fix matrix block numbering so that melting works without compositional fields
add a mesh refinement criterion that ensures a minimum refinement level in dependence of depth
remove Makefiles from repository
change the default values of the melting model to match the values in the papers
Merge branch 'j-dannberg' of https://github.com/tjhei/aspect into j-dannberg

Conflicts:
	source/mesh_refinement/interface.cc
	source/postprocess/visualization/melt_fraction.cc
	source/simulator/initial_conditions.cc
fix some stuff that got mixed up during the merge