Perform contractions using TBLIS

[fevangelista]

This PR will add the ability to perform contractions using the TBLIS library. TBLIS is enabled at compilation by passing the arguments:

-DENABLE_TBLIS=ON -DTBLIS=<tblis installation directory>

• Add TBLIS to CMakeFiles.txt
• Implement contract() using TBLIS
• Perform straight DGEMM calls using BLAS
• Tests run

[jturney]

Nice! One of our grad students has been playing with TBLIS, too, with very impressive performance benefits.

[jturney]

Looks really good!

[loriab]

One morsel of cmake integration you might like to do now is to made a "component" of TBLIS in the ambit cmake. Would be something like replacing shared with tblis and @BUILD_SHARED_LIBS@ with @ENABLE_TBLIS@ in https://github.com/dgasmith/gau2grid/blob/master/cmake/gau2gridConfig.cmake.in#L48-L54 . Then in your personal psi4's you can add find_package(ambit ... COMPONENTS tblis) so that you only detect ambits with tblis.