Ads1x15-adc

Fix gain and multiplexer input when reading differential ADC as described in this issue (1).

Example added:
- ADS_differential

ADC pin selection in **readADC()** and **requestADC()** methods fixed. Input ADC pin `0`, `1`, `2`, `3` for ADS1015 and ADS1115 in those methods will correspond with `AIN0`, `AIN1`, `AIN2`, and `AIN3` respectively.

**getADC()** method deprecated and replaced with **getValue()** method to get ADC value. Getting ADC value in continuous mode no longer wait for conversion ready. To check if conversion ready, one of **isReady()** or newly added **isBusy()** methods can be used.

Examples added in this version:
- ADS_minimum.py
- ADS_read.py
- ADS_read_async.py
- ADS_continuous.py
- ADS_comparator.py

Fix reading differential ADC methods derived from ADS1x15. See issue (1)