Rename MPC to Mehrotra

to avoid acronym ambiguity, as discussed here https://github.com/elemental/Elemental/commit/e5d79492eea280c8819464ca5eadbe14a0f95189#commitcomment-8745685

This builds for me locally, I still need to figure out how to run your tests though. Doesn't look like you have a top-level `make test` or `make check` and `ctest -V .` doesn't find anything.