calibrate for printing mode

[cavamora]

when i start printing here, the scale receives more ˜100g. probably, because of the force applied from pulling the material... whold be great two calibrations.... idle and printing.

[techman83]

It'd be neat to discard readings while filament is extruding and only update on layer switch/during retraction. I don't have super deep knowledge about Octoprint's framework though and not something required for my use case. However a very interesting idea that I'd be more than happy to take code examples/PRs for!