Tidied up delta auto-calibration code. We now use least squares for 3,
4, 6 or 7-factor calibration.
When doing delta calibration, if a probe offset was used to adjust the
head position when probing, use the actual head coordinates instead the
probed coordinates.
Bug fix: newline was missing at end of SD card file list sent to USB
when in Marlin emulation mode.
Bug fix: Heater average PWM report (M573) sometimes gave inaccurate
values when the S parameter was used.
1. Z-probing is now done in two stages: a fast stage at the configured
home feed rate until within 10% of the target value, then a slow stage
at 20% of that feed rate.
2. Provisional support for ultrasonic Z-probe.
3. Added calibration temperature and height temperature coefficient to Z
probe parameters.
4. Z-probe parameters and Z-probe type are now saved to flash memory so
that they survive power-off and reset cycles. Separate parameters are
retained for IR and ultrasonic probes in case both are fitted.
5. Fixed issue with doing slow Z-moves immediately after Z-homing or
probing.
