APE 14: pixel_to_world to accept with_bounding_box

[pllim]

Please see comment from @nden at spacetelescope/gwcs#240 (comment) .

APE 14 specifies that the low-level pixel_to_world_values method may return NaN where the WCS is not defined. This is not mentioned for the high-level methods which is probably an omission. In any case there's no way to pass flags to the high- or low- level methods in APE 14.

[astrofrog]

I don't think we'll be changing the APE anymore, so I'm going to move this to the core repo as it should be clarified in the docstrings for the base classes.