EnviroDIY/Arduino-SDI-12: v1.3.6: Fixed extra compiler warnings

EnviroDIY Arduino-SDI-12 is an Arduino library for SDI-12 communications to a wide variety of environmental sensors. This library provides a general software solution, without requiring any additional hardware, to implement the SDI-12 communication protocol between an Arduino-based data logger and SDI-12-enabled sensors. SDI-12 is an asynchronous, ASCII, serial communications protocol that was developed for intelligent sensory instruments that typically monitor environmental data. Advantages of SDI-12 include the ability to use a single available data channel for many sensors. This work is motivated by the EnviroDIY community vision to create an open source hardware and software stack to deliver near real time environmental data from wireless sensor networks, such as the Arduino-compatible EnviroDIY™ Mayfly Data Logger. New for v1.3.6: Fixed extra compiler warnings: A very minor update to fix compiler warnings found when using -Wextra in addition to -Wall