Adding ECG functionality and various updates

[delpreto]

Adding ECG functionality, options to selectively disable sensors, and updates to file-based commands.

• ECG-related updates include:

  • Adding ECG interface functions including the ADC and the GPIO expander.
  • Adding logging functions for ECG data.
  • Creating a thread for ECG acquisition and logging.

• General updates include:

  • Can selectively enable or disable sensors and acquisition sources; this is helpful for debugging specific components, or for running when sensors are not available.
  • Clear user commands after they are ingested, so they only execute once.
  • Only signal that a new user command was received if the command is not empty.
  • Delay before canceling threads, so they have a chance to stop gracefully.