Selected head changes

This pull request contains the changes required to use the currently selected head, rather than default to E0 for various commands. This is a requirement for the firmware to support multi extruder printing.

In addition the functions which access pins have had the guard if statements updated to use the correct pins for both the Duet and the Duex4.

Tested working on a Mendel90 without bed probing (but no changes made to the bed probing logic).