Allow more control over directions and direction labels

[pjireland]

It would be useful to allow more control over the windrose directions and direction labels. In particular,

1. Provide the option to label by the numerical degree values (instead of always using N, NNE, etc.) - that is, customize theta_labels.
2. Provide the option to customize the theta bins (instead of hard-coding to 11.25 degree increments) - that is, customize theta_angles.