Removes teeny-tiny edges from a tissue. This may be useful if a template or simulation generates a tissue with edges that are very short or approaching zero which could lead to a singularity.
RemoveShortEdges[tissue, threshold] - returns a tissue with all edges of length shorter than (threshold)*(mean edge length).
RemoveShortEdges[tissue] - is equivalent to RemoveShortEdges[tissue, .01]
Uses MergeVertices until all offending edges have been removed.