probe: ProbePointsHelper adjustable minimum points

Factor out minimum required points check for ProbePointsHelper
in prep for setting different values for quad_gantry_level
and z_tilt_adjust

Signed-off-by: John "Fess" Fessenden <fess@fess.org>

klippy/extras/z_tilt.py

@@ -23,6 +23,7 @@ class ZTilt:
         if len(z_positions) < 2:
             raise config.error("z_tilt requires at least two z_positions")
         self.probe_helper = probe.ProbePointsHelper(config, self.probe_finalize)
+        self.probe_helper.minimum_points(3)
         self.z_steppers = []
         # Register Z_TILT_ADJUST command
         self.gcode = self.printer.lookup_object('gcode')