Parallel diagonalizer has to be called as an external application

[ageorgou]

The parallel version of the diagonalizer, PDSYEV, is currently used when the supermatrix is too large for OpenMP to handle. However, it is built as a seperate application, and calling it requires stopping the TROVE workflow.

We should integrate the diagonalizer into this codebase, or use it as a library, so that users can take advantage of it implicitly while calling TROVE as usual.