Add multi signal function to python package

[chinandrew]

Closes #215

Summary of changes:

• Add signals() function to fetch multiple signals for parity with R package.
• Add tests and update documentation
• Add a lint ignore to an argument for aggregate_signals(), which would be nice to change but since it's a released function we may want to be more careful about it. (At least it's not a kwarg)

For now I'm doing this for R parity, though I wonder if it makes sense to just combine them into one function and making signal() private