refactor: PrepBot

[martintb]

The prepbot systems needs to be refactored on a number of levels. In particular, these changes should be made so that much of the mixing calculations can be moved to the OT2Server

• Prepbot should understand the difference between solutes and solvents and obey common lab prep conventions
  • e.g., removing a fixed volume of a solvent + solute mixture
• users shouldn't have to instantiate components. These should be made automatically and inferred.
  • Context manager for mixing calculation?
  • Component properties can come from database or be queried (solvents need density, density is needed for SLD)
• Units should be required, but perhaps automatically added?
• Mass fraction setting should only require n-1 components