Add Ability to Set Individual Thresholds For Each Photoresistor

[Ay1tsMe]

Love the project. I ended up building one myself but because I wasn't able to order the same photoresistor type in big batches, I had to use different photoresistors for each square.

Because they all have different resistant values, I had to adjust the threshold for each individual photoresistor. I've edited the code so that it's easy for users to choose to either set a global threshold or write a matrix for each square.

Hopefully this may be useful for someone who is building one and needs to set thresholds for each square

[Ay1tsMe]

Liboard now can enter "Calibration Mode" if given the c command in a serial monitor. It will then wait to receive either a global threshold value or individual threshold values which update the THRESHOLD variable.

This is used in conjunction with LiBoard/Python#4 to automatically update thresholds on the fly. Both these PR's aim to address this issue #2

[Ay1tsMe]

You can also enter ! command in serial monitor to show the current value of THRESHOLD variable.