2.1.0 release supports both high and low side wiring

Change to add high_side boolean to initializer. This defaults to true and keeps the old behavior of the library which assumes a high side connection (thermistor from ADC input up to high logic/5 or 3.3 volts). However if high_side is set to false the logic changes appropriately so a low side connected thermistor (ADC input down to ground) works correctly too. No functional change for current library users--the default high side parameter will behave exactly as before.