Make MPI into a package extension

[ChrisRackauckas]

It's a heavy dependency, and thus requiring it by default when it's only used by a small subset of applications is not necessary. If it's made into a package extension then I do not think any functionality is lost.

[ChrisRackauckas]

Is Graphs.jl actually used anywhere?

[kunyuan]

Thank you for your suggestions. We will take a look if it is possible to make MPI into a package extension.

[kunyuan]

Graphs.jl right now is used in the MCMC algorithm to check if the integrals are connected through the "neighbor". In the future, we will add a new type of variable, namely Graphs. Graphs.jl will be more useful then. If you have a better idea to handle the dependence on Graphs.jl, please let us know. Thank you very much!