Digital Signal Processing extension - API review and integration

[cybertyche]

The prototype/research implementation of DSP over Trill has been developed, but it is not in product-ready shape. It needs a thorough API review, and a cleanup of the code, as well as tests added.

Key concepts to review:

• DSP relies on a key notion of a "signal", a data stream that is known to have only one valid value per key. It also relies on equal spacing among data points, and allows one to sample or interpolate to achieve it. How this is done needs to be simplified.
• How the above interacts with grouping is also to be reviewed and approved - vanilla grouping in Trill is already a way to establish uniqueness among keys

Research paper: https://dl.acm.org/citation.cfm?doid=3035918.3035935