Clarify dilution in calibration curve

[ewallace]

From conversation with @j-aux.

For primer calibration, it helps to start with a moderate dilution (not necessarily 1x).

We should check, and respond / edit documentation as needed, if the code that we have supports dilution series for calibration curves that start at other values, e.g. 2x not 1x.

[ewallace]

I just looked at this. The code supports it by editing dilution and dilution_nice arguments. I think we can address this by explaining in the documentation string, and it won't take very long to do.