use omp_get_max_threads if setNbThreads is not set

Joerg Buchwaldrequested to merge
joergbuchwald/eigen:fix_multithreading into master

Reference issue

Fixes #2887 (closed)

What does this implement/fix?

This MR restores the behavior documented in https://eigen.tuxfamily.org/dox/TopicMultiThreading.html. Currently, eigen does not parallelize if setNbThreads is not set. The MR uses omp_get_max_threads if setNbThreads is not set.

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