Merge with Homebrew-science

Here is a list of formulae which were changed:
 - petsc  - improve support by adding many external package from homebrew and provide Debug/Opt flavors
 - ann - fix compilation

here is a list of the new formulae
 - gmsh - mesh generator and post-processing in 1D,2D and 3D
 - slepc - standard and generalized eigenvalue solvers
 - feel++ - finite element library in c++