Metis and SuiteSparse

This is a possible solution to https://github.com/mxcl/homebrew/issues/15537. Since I can see that both Metis 4 and 5 are useful and Homebrew doesn't (yet) support versions in formulae (I believe), this may be a case where two versions of a library could co-habitate in two different formulae. The formulae `metis4` and `metis5` `conflict_with` each other. The revised `suite-sparse` formula `depends_on` `metis4`.

Does this look reasonable?