This provides a function that wraps the sbd eigensolver, along with a struct that holds the result. This adds dependencies on MPI, OpenMP, and BLAS. The sbd code generates several warnings, so it is marked as a SYSTEM header in CMakeLists.txt.

Remaining action items:

• Install MPI etc. so CI passes
• Possibly move the fcidump to the C++ source file, so it does not load a relative path (which requires making an assumption about the current working directory)
• Add documentation
• Add tests of eigensolver output
• Can we suppress the output from sbd when the tests are run?
• Add release note