Add metadata to examples that output Signal K numeric values

[joelkoz]

The Signal K specification states that numeric values should have a "units" defined. Review our examples, and if any output a number on a Signal K path that is not already in the specification, add the new SKMetadata to the example to define the units.

[ba58smith]

signalk_output.cpp contains this debug warning message: WARNING - No metadata for %s. Numeric values should specify units. But that's true only if the path isn't already in the Signal K specification. Maybe this warning should indicate that? Something like WARNING - No metadata for %s. Numeric values should specify units, unless this SK Path is already in the Signal K Specification.?