polars-plugin

https://github.com/SignalK/specification/pull/360

Plugin + webapp for sailboat polar handling:

• [x] use plugin configuration to manage polars
• [x] use webapp to display polar curves
• [x] use plugin to derive performance data
• [x] use webapp to track current performance (sort of, very rudimentary)
• [ ] support multiple polars instead of the current hardcoded one, including config ui + http POST to set the active one
• [ ] run/beat VMG sets
• [ ] tack / gybe angle set