Pipette+: Making homing details modifiable at run-time

[Laura-Danielle]

The upcoming Pipette+ models are different sizes than the current pipette models internally, and therefore have different homing requirements compared to current pipettes. This requires changing details about homing, such as

Home position
Motion extents
Possibly microsteping details
Backlash value for pipettes

These are currently set in the smoothie config (i.e. here) and must be made modifiable at runtime. This may involve adding new gcodes and modifying smoothie, or we may find some way to modify this otherwise.

This ticket is done when we know how to do the modifications.

Will be completed in same PR as #3323