* SSE: fix reduce to handle non-null
* add type for data.Field that is float64 or *float64
* resample fix for non-null input case, add couple non-null tests
- Add the ability to stick metadata attached to a value in expressions. Currently uses Frame.Meta.Custom.
- None of this is consumed by anything yet, so an incremental step.
* comes from grafana/gel-app
* remove transform plugin code
* move __expr__ and -100 constants to expr pkg
* set OrgID on request plugin context
* use gtime for resample duration
* in resample, rename "rule" to "window", use gtime for duration, parse duration before exec
* remove gel entry from plugins-bundled/external.json
which creates an empty array for plugins
