PARALLEL_CORES is a cache variable.
Add function to use all available cores.
Always use all cores in python.
Use 'useAllCores' for benchmarks.
Use all cores for tests.
Merge branch 'master' into num_cores_option

Conflicts:
	python/openann/util.pyx
Fix utils.
Merge pull request #5 from AlexanderFabisch/num_cores_option

Num cores option