Add plot_rarefaction and remove plot_SNPie

[dmcglinn]

The new function plot_rarefaction will be useful when one would like to visually compare two or more groups of rarefaction curves without censoring the comparison down to the smallest shared number of individuals as is done in plot.mob_out. Our todo list in #21 suggested we should remove plot_SNPie due to its dependence on rgl.

I also did other minor house keeping changes to the documentation. The function rarefaction returns just a zero if you feed it a vector of zeros.

[dmcglinn]

Also I'm finding the Roxygen2 argument @inheritParams to be pretty useful for importing documentation for common arguments e.g.,(log). We should eventually try to use this more frequently internally within our package so we don't have to update documentation text in multiple places as the description of an argument changes.

[rueuntal]

Hey @dmcglinn does this PR require close review? I'm currently on the plane and won't be able to get to it until tomorrow. Feel free to merge if my input is not essential :)