Added processor timer for greater stability

Changed the incoming data ISR to use a processor timer, this makes the reception more stable, especially when the ISR is controlled by an external library. This also creates some conflicts with other libraries that use Timer2.

Also made changes to the write functions to use the timer to reduce the amount of time that all system interrupts are off.

Finally, forcing all SDI-12 objects to use the same buffer to reduce ram usage.

Better integration inside other libraries

Added notes and an empty constructor/populated begin method to allow this library to be more easily called inside of other libraries.

Fixed timeout values

Fixes the time-out values for the ParseInt and ParseFloat to be -9999. This was the intended behavior all along, but at some point those functions changed in the stream library and the identically named functions within SDI-12 intended to "hide" the stream functions ceased to be calle.

Initial Release

The first "official" release of this interrupt-based SDI-12 library for AVR and SAMD Arduino boards.