Add missing includes in task and section task algorithms

See merge request !2

Improve README and rename to README.md to use GitLab Markdown

See merge request !1

The page is rewritten using markdown format for easier readability.
Markdown '.md' extension is added to the Doxyfile.

This allows altering the LICENCE text without changing all files. It should
avoid merge conflicts in the future.

The load failed silently when using fma files. They are now handled correctly.

Api updated : Error spotted by D Barbier fixed alongside another one, and add a definition of scalfmm_call_delete()

Significant modif in Chebyshev FMM (may impact your library): Provide a variant of ChebM2L without compression (new name FCheb) and use prefix FChebCmp for the compressed variant (former FCheb); Now FChebTensorial does exactly the same thing as FCheb but for tensorial kernel matrices.