DELTA_RADIUS: M666: add R code for radius, store it in EEPROM

This implements M666 R<DELTA_RADIUS> parameter for delta printers (Rostock etc.).

Allows to change DELTA_RADIUS on the fly and save/restore it to/from EEPROM.
This is used for adjustment when Z0 in center is at different height than Z0 near towers (while these are equal to each other).
I think next addition probably should be M666 D<DELTA_DIAGONAL_ROD>.

Notes:

In Marlin DELTA_RADIUS is a calculated value:
DELTA_RADIUS = (DELTA_SMOOTH_ROD_OFFSET-DELTA_EFFECTOR_OFFSET-DELTA_CARRIAGE_OFFSET)

You can get it's calculated value using M503 (look at M666 ... Rxxx).

I theoretically replaced the constant DELTA_RADIUS with a variable, so it should work.
It's only tested with my delta printer (Rostock variant).

related to #719