Implement faceting by two variables

[ThomUK]

The current implementation of faceting accepts only one column name, and faceting is implemented using facet_wrap.

It would be convenient to be able to facet by two columns, and to implement using facet_grid instead.

This could be via:

1. Changes to the main ptd_spc function
2. Addition of a convenience wrapper function which makes unfaceted calls to ptd_spc, and then arranges the resulting plots using eg. {cowplot} or {patchwork}.

This issue is low priority because it's possible to do this manually either with cowplot or patchwork with a little effort. Raising an issue mainly because I needed it today, and to log the possible addition of functionality.