isobands()/isolines() methods for data frames and SF objects

[mpjashby]

Currently isobands() and isolines() take three vectors, x, y and z. Particularly for use in pipelines, it would be useful to add:

1. A method that instead took a data frame/tibble and the names of three columns that contained the x, y and z values.
2. A method that took an SF object (from which x and y could be extracted) and the name of a column containing the z values.

The second case could alternatively be implemented by also extracting the z values from the SF object, but that would be less flexible for the case where the x and y co-ordinates were stored in the SF geometry column and the z value (perhaps not representing elevation) was stored in a separate column.

This is a shameless feature-request issue rather than a PR, but I'm raising it to get the maintainers' thoughts on whether this is a good idea or not.