Doser functionality (loose ends)

[catbarph]

These probably need to be split into separate issues, but for now, a brain dump...

• peristaltic pump rate measurement, for calibration purposes, by timing a few trials of full liquid transfer from a small vessel of known volume
• pH balancing function - tune gains manually and increase delays (work on "auto-PID-tuning"?)
• nutrient mix dosing from more than one pump according to desired A:B ratio of nutrients (to be determined by end-user/fertilizer spec.)
• account for poor diffusion/mixing in the reservoir by delaying repeated additions or otherwise waiting for positive feedback?