Add thread count variant to atlas (#9999)
Add thread count variant to atlas (#9999)

* Add thread count variant

Atlas automatically configures itself and optimizes to run with a fixed number of threads. This can be overridden during compile by setting a flag and atlas will tune itself to use the specified number, rather than all system CPUs. 

Default value is the existing logic, autoconfigure.

* Fix Linting

* Another lint for longline

iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack