Simplify public interface

[RastislavTuranyi]

Currently, we have a lot of individual functions that do similar things - these could be merged together into one function with a argument that would cause one of the original functions (maybe in the lapack style) to be called. Doing this should simplify the API, hopefully making it easier to use.

Candidates for this:

• allocate_tree and allocate_tree_monolithic -> allocate_hodlr
• multiply_hodlr_dense and multiply_dense_hodlr and multiply_hodlr_transpose_dense