[WIP] Add simple visualization for Joint-RPCA networks

[gibsramen]

Modifies main Joint-RPCA code slightly to now save a mapping of which feature is from which modality (table). Adds some code to create a simple network using NetworkX.

@cameronmartino What do you think of the proposed changes to the main Joint-RPCA code?

TODO:

• Add QIIME2 support
• Discuss user-input of table names
• Handling for duplicate IDs (i.e. same feature name in multiple tables)
• Plotting options
• Update tests
• New tests
• Documentation updates/additions

Eventually I think it would really cool to have an interactive 3D network using something like Plotly but that's a whole new project.