Refactor stat computation methods

[hadley]

Many geoms need to alert the user about default choices. It would be convenient to have a standard inform_defaults method that prints once per stat (not once per group or once per facet). This would provide a consistent approach to informing the user, rather than the ~3 different approaches currently used.

[hadley]

Should inform_defaults actually be updated compute_params? Would change behaviour slightly (params currently computed per panel), but would create more consistent output, and messages could be more informative.

Is it better to make compute_groups() more explicit by pulling out into own function that each stat much then call?

[hadley]

• inform_defaults() -> compute_params()
• calculate_groups() -> compute(), calculate() -> compute_group()
• In boxplot and smooth, replace calculate_groups() with compute_params()