Implement Data Upload/Charging Mode in ceti-tag-data-capture systemd service

[MattCummings0517]

This is to address the following operational bug that has been occurring in the field:

• Tag is recovered and brought to shore for data upload and charging. Usually the Tag is in sleep mode at this point which means the battery pack has been disconnected internally from the electronics and the system shutdown gracefully
• Operator attaches the tag to a charger and it powers up, but batteries are still disconnected internally, so it is running from charger power only at this point.
• Operator connects by WiFi and wakes the Tag which connects the batteries, which then start charging. This should be confirmed by the operator by observing the charge current or otherwise.
• Operator, per instructions, stops the ceti-tag-data-capture service because we don't want to be recording data at this point
• Operator starts uploading data while charging and leaves the Tag unattended. After the batteries charge up the charger turns off, but the Tag is still on. If left in this state (service stopped, running from battery only), the battery monitor system will not work (because the service is stopped), and one cell will blow through the 3V limit and go all the way to 2.6V where the hardware safety catches it.
• Now it is very hard to get the Tag to a good state because the batteries are out of balance AND the system keeps trying to shut itself off after a restart because of the low battery condition

PROPOSED FIX We'll add a new mode to the ceti-tag-data-capture service for charging and upload. The operator instructions will change for Tags with this fix in place. Instead of stopping the capture service, the operator will invoke the new mode. The new mode will simply gracefully stop the recording threads gracefully and provide and indication via one of the LEDs that it is in this new mode.

[MattCummings0517]

Closing this issue - was addressed by adding an independent battery monitor service. This service runs separately from the main recorder app and will handle the case where batteries were being overdischarged.