[mqtt.homeassistant] fix multi-speed fans

[ccutrer]

The original implementation for a fan with speeds was written without an actual device, and just guessing from Home Assistant's limited documentation. Now I actually have a 3-speed fan, and I can see how it's expecting to behave. In short, "percentage" is actually "speed", and it just defaults to a range of 0-100, but if it advertises as say 0-3, it actually wants 0-3, not 0, 33, 66, and 100.

• fix step math so that the state description represents the step scaled to 0-100%
• introduce a format override to PercentageValue - Home Assistant expects integers in several places. for a true percentage, most devices were accepting a floating point just fine, but if there's a device (like a 3-speed fan) that has a command template that translates the speeds to some other string value, and uses a lookup to do that, it's important that the value we send to that template is an integer
• fix that the value template for fan state is state_value_template, not value_template like almost everything else

[ccutrer]

@openhab/core-maintainers: is there any guidance on the preferred way to model a multi-speed fan (with discrete speeds between 0 and some number less than 100 - usually, between 2 and 10)? I see four options:

• As a Dimmer. This has the advantage of being able to accept ON/OFF commands, with ON restoring to the previous speed. A state description could theoretically provide a step value to make any sliders behave better (though I seem to recall that at least one of the UIs don't handle a step value on a slider very well). This is how my ZWave fan controllers present themselves, but I think that's just how ZWave handles it (multi-level switch control command class), and less a conscious decision on the part of the openHAB ZWave binding, or zwave-js. It's also how HomeKit handles Fan accessories (you can set min/max on it to limit to discrete speeds, but it gets weird in the UI sometimes, and Siri's spoken interpretation pretty much assumes percentages). There can also be weirdness with a Dimmer - my ZWave fan controllers are 4 speed, but because ZWave presents it as 0-99 (and 100 is return-to-previous-value), speed 1 is 24.24%, speed 2 is 49.49%, speed 3 is 74.75%, and speed 4 is 100%. This PR essentially implements this option. For myself, I like the discrete numbers, and have rules to maintain number items calculated from the dimmer items, and use the number item for my UIs.
• As a Number. 0 is off, and speeds map directly - 1, 2, 3 etc. This could be harder to expose to some UIs, especially HomeKit. You also have no way to send a command turning it on returning to whatever speed it previously was.
• As a Number and a Switch. This duplicates things slightly, but gives you an ON command. This is how [wiz] Initial contribution #17681 that I'm working on currently models it for WiZ devices (as it's a direct mapping from the underlying API). This also matches most closely with how Home Assistant's discovery information for MQTT models it. Though I get the impression Home Assistant itself maps it to something more closely resembling a Dimmer. It is a little unclear if the Number should go to 0 when the fan is off, or if it can be used to always show the what the speed would return to when it gets turned back on.
• As a Number and a Dimmer. This would give the most flexibility, but with the most duplication of information. Use the Dimmer when you need something that deals better with percentages. Use the Number when you want something that better deals with discrete speeds.

The latter two have a little bit of precedent when dealing with a light bulb that supports dimming (or color) and color temperature. My experience with those things is it can be very inconsistent if the color temperature channel goes to NULL or UNDEF when the light is OFF, or if it stays at its previous value (and what it would return to if it turns back on). It's also inconsistent if sending a command to the color temperature channel will turn the bulb on if it's currently off.

In a slightly different way, there is precedent with color temperature to model as a Dimmer and a Number.

So... thoughts?

All of these questions could probably use some explanation and reasoning in the FAQ at https://www.openhab.org/docs/developer/addons/faq.html#modelling-channels.