Various changes to get working on instrument

Added wash at beginning
Choose DITI tip based on mixing volume if mix is indicated
Added vector commands to move plate back to sample position after PTC
steps
Use two liquid class variants: -First, -InLiquid
Fixed volume checks
Added worklist.wash
Only for vector commands, grid position is 1-based, not 0-based
Added prefill to trp1.py
Added periodicWash