Implement handling of NaN values

[andig]

SMA describes NaN values for sunspec types in https://www.sma.de/fileadmin/content/landingpages/pl/FAQ/SunSpec_Modbus-TI-en-15.pdf, chapter 3.6. I'm note sure if those are part of the SunSpec spec, however if they are it would be nice to add them to Point and return NaN instead of the literal nan bytes value.

[andig]

I've solved this by rolling my own scaledValue function:

func scaledValue(p sunspec.Point) float64 {
	f := p.ScaledValue()

	switch p.Type() {
	case "acc16":
		fallthrough
	case "uint16":
		if p.Value() == uint16(math.MaxUint16) {
			f = math.NaN()
		}
	case "acc32":
		fallthrough
	case "uint32":
		if p.Value() == uint32(math.MaxUint32) {
			f = math.NaN()
		}
	case "acc64":
		fallthrough
	case "uint64":
		if p.Value() == uint64(math.MaxUint64) {
			f = math.NaN()
		}
	case "int16":
		if p.Value() == int16(math.MinInt16) {
			f = math.NaN()
		}
	case "int32":
		if p.Value() == int32(math.MinInt32) {
			f = math.NaN()
		}
	case "int64":
		if p.Value() == int64(math.MinInt64) {
			f = math.NaN()
		}
	}

	return f
}

Would appreciate any insight if this is SunSpec standard (and should serve a PR) or if it's SMA-specific.

[andig]

Actually, the are standard: Information Model Overview - Version 1.8, section Data Points / Standard Data Formats.

[andig]

See sunspec/pysunspec#74 for discussion related to the python reference implementation.