add pth_creator()

[ijlyttle]

This would be an S3 generic that would act similarly to pth_transformer(), in that it would return a function that could create a pth_mat using a given color space.

I think this would be cleaner than the current pth_mat_replace_data(); I would look to deprecate that...

[ijlyttle]

We can also use this to simplify the `[`.pth_mat() function - this can be done once for all pth_mat using the dispatch for the pth_creator() function.