print reminder about tubes and buffer before plate maps in `Mix.instructions`

[damienwoods]

If include_plate_maps=True in Mix.instructions(), it would be good to also print a line indicating the number of tubes that need to be pipetted from, plus one pipetting step indicated as the buffer. Maybe as the first, or last, line. E.g. "25uL of buffer, plus 2uL each from 43 tubes to be pipetted."

[dave-doty]

Did you really want just the number of tubes?

There's two options that make sense to me:

1. Minimal information: print a reminder between the mix table and the first plate map (or the bottom of the whole thing) that tubes and buffer should be included. The user knows to look at the mix table to see exactly which tubes, how much of each, and how much buffer.
2. Maximal information: print the names of actual tubes to be pipetted, with volumes, and the volume of buffer.

[damienwoods]

For me the example text I gave is fine.