Adafruit_ASD1015.cpp: fix or-ing twice with same value
Adafruit_ADS1015: add writeConfigRegister helper
Adafruit_ADS1015: getLastConversionResults: disable delay in continuous mode

Add m_continuou shelper for tracking configured mode, and use to disable
delay in getLastConversionResults when in continuous mode.
Adafruit_ADS1015: fix comment on readRegister
add ADS1115 DR mask and codes, different rates; use as configured rate for all configurations
add setContinuousMode