Add ability to set TLU trigger inputs threshold in V

TLU trigger inputs thresholds are controlled via 2 DACs. It'd be nice if thresholds could be set in units of Vs, rather than using raw integers being written into the DAC registers.

DAC datasheet: https://www.mouser.co.uk/datasheet/2/609/AD5625R_5645R_5665R_5625_5665-877198.pdf