Update interface of SigPro w/pipelines and primitives

[andyx13]

Larger update:

• Add Primitive and Pipeline base classes (Issue Update primitive interfaces #51)
• Added contributing_primitive and basic_primitives module to assist with new primitive creation/usage
• Added tree and layer pipeline creation (Issues Syntax for pipeline creation #41 and Support for more complex pipeline architectures #52)
• Additional testing for pipelines
• Additional documentation and demo notebooks for new pipelines (Issue Demo Notebooks for Pipeline usage #55)

Additional modifications:

• Fix some existing primitive JSONs (Issue Incorrect classification for stft.json and stft_real.json #53)
• Demo fix (Issue Loading demo dataset at random index #35)

[andyx13]

Made an additional slight modification to add a primitive_inputs argument. This is necessary so as to not break the FrequencyBand primitive, which has a non-standard input argument.

[sarahmish]

Great effort @andyx13! There are minor comments overall, and some improvements to be made to the pipeline initialization.

[sarahmish]

Looks great @andyx13! Few comments to improve readability of the code.

I also noticed that the unittests are missing cases where the pipeline would raise an error. I think it would be good to include these cases as well!

[sarahmish]

Ready to be merged! Great work @andyx13!