rename all references to 20000 uL tubes as 15000 uL tubes

Due to a mistake very early in the code development, the 15 mL (15000 uL) tubes are incorrectly labeled as 20000 uL (20 mL) tubes throughout the code. This is particularly bad because we have 2000 uL (2 mL) tubes so it's far more likely that those get confused compared to 2000 vs 15000 labels. I hope this is as simple as some search+replaces.