Integrate H-Mat support via hmat-oss
This PR adds the support for H-Matrices in the coreblas kernels via the hmat-oss library (https://github.com/jeromerobert/hmat-oss)
@all: Please have a quick look to let me know if something seems wrong. I keep it as WIP for now until things have stabilized.
Edited by Mathieu Faverge