Update draw.basis so that it works for bivariate (and higher dimension) smooths

[gavinsimpson]

Need plotting code for

• bivariate smooths
• Make draw.basis() work for trivariate smooths #258
• Make draw.basis() work for quadvariate smooths #259

[gavinsimpson]

There's also the additional problem of what to do with factor-by smooths in this setting. Could facet on ~ .by + .bf? But then what happens with the labeller (prefix_label_both())?

At the moment, if there is a factor by associated with the smooth the code just facets on the by variable and not on the basis function, so you'll get multiple surfaces on top of one another, it it works at all.

Might just be best to not draw the basis functions for each level of the .by factor - they're all the same anyway...