Feature/delta segmentation

Added delta_segments_per_second to match what delta/Marlin does for delta bots.
This provides a segmentation based on the current feedrate and speed override, where the number of segments is inversely proportional to  feedrate.

Added relevant config option and example.

Allow mm_per_line_segment to be set to zero to disable segmentation altogether.
NOTE that I changed the default to 0.0 if this is not defined in the config file, if this could cause shock and awe then change the default back to 5.0
NOTE if both mm_per_line_segment and delta_segments_per_second are defined in config and delta_segments_per_second > 1.0 then delta_segments_per_second is used.