feature request: plot a subset of positions

For the accuracy analysis (maybe others?), it would be convenient to be able to group different positions. For example, if we wanted to plot 1 bar each accuracy for early, middle and late positions, you'd have to do an spc, and then manually select the positions. we could have a `posgroup` argument that allows you to run a separate analysis for multiple groups of positions. e.g.

`quail.analyze(egg, analysis='accuracy', listgroup=['average']*16, posgroup=['start']*3+['middle']*10+['end']*3)`