Added More Flexible Partitioning and Subgraph Creation

[JoeyT1994]

This is in response to issue #28 .

I have updated subgraphs.jl to allow for increased capability.

1. The subgraph function (which creates a data graph which merges certain vertices of an underlying graph g) has been replaced by create_subgraphs which has 3 versions. One where you specify the #vertices_per_subgraph you want, one where you specify the #subgraphs you want and one where you explicitly provide the vertex groupings you want as a dictionary.
2. This function is aided by the partition_vertices function which forms vertex groupings (backending onto the partition function from KaHyPar or Metis) based on either a specified nvertices_per_group or ngroups.

[mtfishman]

Thanks for the PR @JoeyT1994. I was actually in the process of making similar changes in #30, sorry for not letting you know before you made this PR! Could you take a look at the proposed changes in that PR? I might still tweak things a little bit but the basic idea is there.

[JoeyT1994]

Thanks for the PR @JoeyT1994. I was actually in the process of making similar changes in #30, sorry for not letting you know before you made this PR! Could you take a look at the proposed changes in that PR? I might still tweak things a little bit but the basic idea is there.

No worries! I will take a look over that as that seems to contain what I added here and more. Shall I close this PR then?

[mtfishman]

I left a comment with details about the new interface here: #30. If that looks good to you feel free to close this.