Add function: relative two norm difference of vectors
Add appropriate permutation of vectors in H-matrix multiplication

* H matrix self test bascially finished now
Properly generate/remove git_version.h with make
Move Hmatrix code into main micromag library.
Incorporate hlib into oomph-lib build machinary
Fix bug where h-lib bem matrix sign was opposite to mine

* Problem was just that the nodes (within each element) were in the wrong
  order when fed into hlib.
Clean up code/comments in hlib matrix test
Clean up code/comments in hmatrix.h

* Null Cluster_tree_pt in constructor
* Delete Cluster_tree_pt in destructor
* Remove debugging code (scrambling of the order of some mesh inputs)
* Clean up comments
Add cli argument and dummy build function for H-matrices in BEM handler
Use pointer for bem_matrix storage in bem_handler

Because we need to be able to handle Hmatrix now as well as a dense matrix.
Remove nmin argument from HMatrix constructor
Add sphere energy self test for hierarchical matrix BEM