data logger arduino code updates

Allows Arduino to function as a callable temperature sensor and a data
logger. Only Arduino code updated - existing Python and Igor code will
not work with these updates. Python updates to follow.
class_sensor.py updated for RPi serial address
data_logger_app potential changes

Adds in matplotlib.dates - not sure what this achieves but will leave it
in temporarily.
class_sensor updated for latest arduino code

Start and stop serial write commands added to the start and stop
monitoring commands to set the data logger for periodic measurements.