Add a basic confidence test to detect open cell sense cable connections

(This feature is not quite ready for release, but review and testing is welcome...) Add LTC68042configure_basicConfidenceTest() to key_handleKeyEvent_off() to use the cell balance circuits to check that the cell sense cable connections have continuity. The test adds ~51 ms to the duration of key_handleKeyEvent_off(). This increases the latency for detecting a key-on event after a key-off event is detected. The existing latency is ~30 ms, and now it is ~80 ms. There is evidence that there is roughly 180 ms available (sample of one...). This change also adds functions to LTC68042cell.cpp to support allowing cell balance circuits operation while cell voltage measurements are underway.