Allow infinite and NA values to be swarmed (second version)

[billdenney]

Fix #19
Related to eclarke/ggbeeswarm#87

This PR uses the new, internal xToFinite() function to convert NA, -Inf, and Inf to finite values so that they can be swarmed.

It replaces #20 with fewer changes.

[aroneklund]

Thanks! This seems to work as expected for the "swarm" and "compactswarm" methods, but has no effect on the other methods ("center", "hex", "square"). Right?

I imagine most people use this package for the *swarm methods. Nevertheless, I would prefer if infinities were treated the same way regardless of method.

I hope to have more time next week time to dive into this..

[billdenney]

You're right that it would not affect the other methods ("center", "hex", and "square"). And, I understand the goal to make them all work the same way.

Thanks for considering it.