Mitos shouldn't have hard MPI dependency

[daboehme]

Apparently MPI is only used in the MPI example, not the library itself, which hardly justifies making MPI a required dependency.

[alfredo-gimenez]

In the develop branch, the library hooks include MPI hooks: https://github.com/LLNL/Mitos/blob/develop/src/mitoshooks.h#L16

This could be decoupled however, in the sense that the hooks not be compiled if MPI is not found through cmake.

[alfredo-gimenez]

Fixed with 576762e