Add raw data logging to sdcard

[oxothnk423]

This PR adds basic functionality to store raw sensor data into a .csv file on SD card.

Use the flag at the top of log.cpp to enable or disable:
bool DATAFILE = true; // set to false to disable data logging to SDcard

The data file is created when the vario timer starts running, and is closed & saved when the timer is stopped. Data will only be captured while the timer is running.

Data is stored from 3 sensors:

• barometric pressure (NOT altitude), every 50ms, at the end of baro_calculatePressure()
• IMU acceleration (total g-load), every 100ms, at the end of imu_update()
• GPS lat, lng, alt (m), speed (m/s), course/heading (deg) inside gps_update()