Generate "pick list" for loading a qPCR plate automatically

[ewallace]

To load ingredients to a qPCR plate automatically, we need to specify reagents and amounts in a source plate to be loaded in the destination plate. This is called a pick list. To enable this, we need appropriate functions and a vignette in tidyqpcr. @j-aux is planning some experiments for this soon, so we can work with her to implement.

This issue is related to, but distinct from, #5. Pick lists would be useful even for loading smaller (e.g. 384-well) plates.