Fix IR and RED values interpretation from Arduino

[RVillani]

Hi, folks!

I'm using your shield and scripts for a medical application. Thanks a lot for them!

I've fixed a bug on the final SpO2 value displayed on the Processing software.

IR and RED values are sent from Arduino in that order and were being read as RED and IR by the Processing script. That changed the final SpO2 value to about 75% in a healthy person, whereas the standard is about 95%. It was also inverting the variation on the oxygen levels, eg., by holding your breath the value was raising instead of lowering as it should. Now it varies as expected.

[VenkateshBhat]

Hello Rodrigo, Thanks for your contribution, we truly appreciate your time and effort to make the algorithm better. We will review the code and merge it. Thanks again,

[RVillani]

Thank YOU guys for making it open source ^_^