adding MAX31856 support

[yoxcu]

This adds MAX31856 support, the module supports different types of thermocouple.
By defining any TCTYPE (K,J,S,...) in PIDKiln.h

#define MAXTYPE1 MAX31856_TCTYPE_J

adafruit_max31856 lib is imported for reading the temperatures.

[Saur0o0n]

I took a quick look - have to think on it. I'm not a huge fan of this many #ifs - especially big #ifs in termocouple readout function. Perhaps it would be better somehow to create single function for each thermocouple type and as parameter just make it read A or B thermoucouple - and call proper one. Or even dedicated files for each max types and use only one.. just thinking out loud...

[yoxcu]

yup did not want to change too much code wise.
8171d65 seperates the code of both boards into two .inos. this should also make it easier to add different boards.
I only have MAX31856 available for testing, this works. I also tried mixing 55 and 56 which also worked as expected (55 threw an error 5 times and program aborted)

[yoxcu]

also is it by design that the int_temp is the mean over both TC CJ?

[ajunca]

Hi, I'm also interested on this feature. What is the state and what needs to be done to be approved?