Find the level of a trixel from its htmid. The level indicates how refined the triangular mesh is.
There are 8*4**(d-1) triangles in a mesh of level=d
(equation 2.5 of Szalay A. et al. (2007) “Indexing the Sphere with the Hierarchical Triangular Mesh” arXiv:cs/0701164)
Note: valid htmids have 4+2n bits with a leading bit of 1