[AP][Solver] Enabled Parallel Eigen

[AlexandreSinger]

The Eigen solver has the ability to use OpenMP to run the solver computations in parallel. Made the AP flow use the num_workers option to set the number of threads that Eigen can use.

VPR did not have the ability to build with OpenMP in its CMAKE. Added an option to the CMAKE to allow the user to enable OpenMP.

Resolves #3030

Currently running the VTR circuits using 4 threads, will show the results. As a preview, MCML's Global Placement went from 141 seconds to 92 seconds on 4 threads.

I made OpenMP disabled by default for now since only the AP flow uses OpenMP. Perhaps we can enable this by default only when AP is enabled or just always enable it.

[AlexandreSinger]

@AmirhosseinPoolad You may find this update interesting since I added a CMAKE option which enables OpenMP.

[AlexandreSinger]

Results on largest VTR circuits:

	OpenMP Disabled	OpenMP Enabled 1T	OpenMP Enabled 4T
GP run time	1	0.9956934236	0.6646688318
Total run time	1	0.9909095635	0.8437623927

Turning on parallelism for 4 threads reduced the overall run time by 15%. I think this number can be even larger with bigger circuits such as the Titan circuits.

[AlexandreSinger]

@vaughnbetz This is ready for review. It looks like this does not have perfect scaling, but it may scale better to larger circuits. Perhaps this can be explored more in a future paper.

What do you think about having OpenMP disabled by default? I could have it enabled by default and then disable it if it cannot be found on the system; or only turn it on if Eigen is installed. I do not think having OpenMP enabled and not used will have any affect; but I wanted to see what you thought.