Make various geogram functions thread-safe

[jdumas]

By "thread-safe" here I mean "being able to call geogram functions concurrently". E.g. being able to run Delaunay triangulation on two meshes in parallel (which can happen when integrating Geogram functions in a node-base application like Blender). This is why having a central scheduler (like OneTBB) helps a lot. Currently a lot of geogram routines use global static variables. Using thread_local instead would help. Some examples:

• CVT.h
• lbfgs_optimizers.cpp
• HLBFGS/LineSearch.cpp
• HLBFGS/ICFS.cpp

Ideally this should be tested with ThreadSanitizer by calling Delaunay triangulation on a few meshes in parallel...