Support for different persistent homology backends

[marcusblake]

As stated in the roadmap, currently, guddhi and giotto-ph are used for various purposes. We want to be able to allow the user to specify which backend to use, and also provide sane defaults if the user does not care/has no preference.

[Pseudomanifold]

I think this is a great idea and would leave this as a potential enhancement. However, I currently do not see what the advantages of choosing between the two backends would be. Issue #7 discusses a specific backend for GPUs, which, if implemented, would indeed prove advantageous to users.