Provide a non-pipelined cell data acquisition mode to optimize test (BIST) code. (aka FORCE_TRIGGER)

All cell data is acquired and processed via calls to LTC68042cell_nextVoltages(). The prior code is well optimized for continuous cell data acquisition, where the next acquisition is started as soon as all of the data from the current one has been gathered, and that new acquisition then runs in parallel with the data processing step. However, for BIST code where conditions are changed for test purposes, and the test then needs to do an acquisition to look for effects, two full acquisition cycles would be required: the first to gather and process (and then discard) the data from the "too early" acquisition, and then another cycle to gather and process the desired cell data. This is inefficient. A new mode argument is added to LTC68042cell_nextVoltages() to allow operating as before (“TRIGGERMODE_ROUND_ROBIN” mode), or to short-circuit any prior cycle as quickly as possible (“TRIGGERMODE_FORCE_TRIGGERED“ mode), and then to proceed with a single direct trigger, wait, gather, process acquisition cycle (“TRIGGERMODE_TRIGGERED” mode). Transitions between LTC68042cell_nextVoltages() modes are supported at any time.