Add support for output_pin and fan templates

[KevinOConnor]

This PR adds support for specifying a math formula for output_pin and fan_generic objects. This works similarly to the "led template" support - one can specify a [display_template my_template] config section that contains a Jinja2 macro providing a formula, and that formula will then be evaluated periodically to determine the pin/fan setting to apply.

The settings from the formula do not pass through the g-code command queue, so the updates are applied even during homing and other long running commands.

This should make it possible to define more flexible pin and fan control schemes.

-Kevin

[bevanweiss]

Perhaps display_template could get a secondary name of just template?
It feels very confusing to be connecting an output pin / fan to a 'display_template' when the intent is to not display anything, but to have it all work in the background...

Maybe dynamic_variable or dynamic_value or something else might be a good secondary name. Just something that doesn't tie it specifically to a 'display' usage.

[KevinOConnor]

Thanks for reviewing.

I agree that display_template is a confusing name. Roughly I'd like to introduce something like macro_template, make render() always available from Jinja2, and deprecate display_template. That change seems orthogonal though, so didn't want to complicate this PR with a config rename.

-Kevin