Add public API for WCSAxes to retrieve the tick locations and labels on each axis

[astrofrog]

I am interested in being able to plot world coordinates in non-Matplotlib viewers, for example in bqplot. However, I would prefer to not have to re-implement all of WCSAxes in bqplot, and an alternative would be to internally store a WCSAxes instance and set limits on it, then query it for tick locations and labels. However, I don't think we have a proper way at the moment to get this - and:

ax.xaxis.get_ticklocs()
ax.yaxis.get_ticklabels()

is not correct because it is what the ticks would have been in the absence of WCSAxes. We might want to consider having e.g. CoordinateHelper.get_ticks() and CoordinateHelper.get_ticklabels() for this.

[astrofrog]

It might be an option to also consider splitting out the 'core' WCSAxes algorithms for determining tick locations, labels, and gridlines so that they can be used in a frontend-agnostic way, which would make the code more future-proof and broadly useful.

[pllim]

Hmm, do we need APE for this? After all, you did an APE for wcsapi.

[pllim]

Also... Does this overlap with the "unified API" philosophy going on over at Astrowidgets, @mwcraig and @eteq ?

https://github.com/astropy/astrowidgets

[astrofrog]

Regarding an APE, I think I first need to prototype things to see how feasible this is in the first place in terms of splitting out the core stuff

If we do just provide a couple of methods on the current WCSAxes to get ticks and tick locations then those would naturally live on CoordinatesHelper.get_ticks and CoordinateHelper.get_ticklabels since we already have setters there.

[neutrinoceros]

This looks related to other existing issues: #14047 (comment)