Minor changes to z-hop min floor logic & Prusa style stamping option for tip forming

[ningpj]

Suppressed z-hop min floor when park moves aren't defined for the mmu sequence to keep nozzle on purge block during toolchange. e.g. when the user has used variable_park_toolchange : -1,-1, 0, 0, 2

Also incorporated Prusa style stamping moves as part of tip form macro in cooling move loop (only stamps on even moves). Controlled using 3 variables: use_stamping (true | false), stamping_insertion_distance mm & stamping_insertion_speed. Currently using unloading_speed_start value for the retraction rather than creating another parameter. Need to check and review move offsets as stamping_insertion_distance "should" technically be from the middle of the cooling tube according to Prusa slicer

Still testing so not ready to merge.

Fixes #486

[moggieuk]

Happy to add this when you are complete. Can you ensure the defaults in FORM_TIP macro default to turning stamping off in the absence of the variables defined in mmu_macro_vars.cfg. That way I don't need a version number change to force users to do the "upgrade install". I.e. everything will work with a simple update-manager refresh. Users who want the functionality can of course run ./install.sh to upgrade. The reason is that I've had a lot of forced updates lately and I have v3.0 coming very soon..

[ningpj]

Seems to be working fine. I've kept stamping distance consistent with prusa's definition which is the cooling tube position rather than the top of the tube which is used by skinnydip. I wanted to keep it consistent for peeps not using it in a standalone manner to confirm slicer settings. I also undo the HH ooze retraction at the start of tip forming as ramming, stamping, and skinny dip all need to reliably extend back into the melt pool. With the retract in place as it was ramming is ineffectual unless you rammed significant volumes to allow the filament to reach the melt pool. People who use tip forming and have manually increased ramming and skinny dip distances after the ooze retraction was incorporated will need to reduce and retune them. Unsure what affect the initial HH retraction will have on the shape of the tip as this is a difference between standalone and slicer tip forming processes. in my case its a 2mm retract and extrude in fairly short order.

I also added an adjustment when the calculated top of the melt pool is less than the slicer enforced 15mm and a warning to increase the cooling tube position. I considered allowing users to override the 15mm distance when operating in standalone mode for v.compact hotends but haven't done so as yet.

ignoring the z-hop when not parking works as expected.