STM32 GPSDO Version v0.04f

First release in the v0.04 series.
Implements tunnel mode command and operation.
OLED display is now a configuration option, as are all the sensors and the Bluetooth interface.
The u-blox Neo-M8 GPS receiver is now configured for optimized stationary antenna operation and increased "quiet time".
Work started on calibration process, command is implemented,
ongoing work on calibration algorithm.
16-bit PWM DAC tested and approved will replace 12-bit I2C DAC as
default hardware to generate Vctl in future versions.
Improved comments and variables names.
Improved documentation.