LED indicator for basic multi-channel recording

[leewujung]

• [x] LED indicators on the board are programmed to show successful multi-channel recording in progress
• [x] Documentation for different LED indicator patterns

[aidanjohnson]

As of now (2018-07-03 at 16h00), the LEDs indicate whether each channel is currently in the process of reading/writing (on for active and off for inactive). The lights turn off if the exit routine (defined by the protocol in the code) is engaged, particularly what we will call a fatal error (e.g., invalid and excessive samples per channel set by the samples per file parameter).

[leewujung]

e.g., invalid and excessive samples per channel set by the samples per file parameter

This can be addressed in UI. Adding as a requirement in #14.