Support for 3 wire RTDs

[osterwood]

It may be advantageous for some applications to support 3 wire RTDs. This requires connecting pin 6 (FORCE2) of the MAX31865 to Pin 5 (FORCE+) instead of to GND. Right now there is do way to make this change due to boards' layout and the Pin 6 GND connecting existing underneath the IC.

The layout could be changed in the future to add a solder jumper allowing pin 6's connection to be selected by the end user.

[osterwood]

As an interim step, two adapter boards have been designed which allow connection of 2 wire RTD (or 3-wire in 2 wire mode) via 5mm screw terminals.

It looks like the error introduced by this is not too bad (0.13C at 100C) and with a 3-wire RTD leg resistance and be accurately measured and error compensation done in software inside of the MCU.