Added checksumFailurePin

[PaulZC]

Hi Nathan (@nseidle),
Currently, processUBX uses Pin 2 to help diagnose checksum failures. This could cause problems for anyone using Pin 2 for other stuff - including SoftwareSerial in my case! ;-)
I suggest:

• Adding checksumFailurePin to the header file.
• Changing the digitalWrite in processUBX so it uses the checksumFailurePin (if defined) instead of having it hard-coded to Pin 2.

Instead of being const int, maybe checksumFailurePin should just be a regular int so it can be changed by the code in your .ino instead of involving editing the header file?
Or, you could really go to town on it and define a setChecksumFailurePin() function to change it?! But perhaps that's overkill...? ;-)
Enjoy!
Paul

[nseidle]

Good catch. Yea, that was a vestige that I failed to remove. A like your new fix.

I don't think we need to expose it via a method. It's really only for debug.