Transitioning from mesh.xml to mesh.xdmf, from dolfin-convert to meshio

See this gist, before converting to XDMF you’d need to “prune” all lower-dimensional elements, as currently FEniCS does not support “mixed” mesh composing from several element types (may they belong to a same dimension or not!) According to their recent released roadmap, the support for mixed elements is for long term…