Add support for minor ticks in 3d axes.

[anntzer]

The actual implementation is shorter than deleting the copies of "Minor
ticks are not supported" in the various docstrings...
Edit: Just a bit longer now that minor ticks are style differently.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[WeatherGod]

I am surprised that it is this simple to get the feature roughly working. Maybe the api has gotten better at this than when I last looked at this. I still have a vague recollection that I encountered a deeper problem when I looked at this 8 years ago.

I do want to try and get this to look right, though. Right now, the minor/major ticks can't be distinguished, and the text labels all overlap each other. The code for handling tick marks and labels all assume major tick semantics. We might need to make a few more changes to get this to look "right".

[anntzer]

There isn't much to be done about overlapping text (you'd get the same with long labels in 2D). I can probably switch have now switched minor ticks to use a length of xtick.minor.size and a linewidth of xtick.minor.width, but that's all the low lying fruits I can think of.

[anntzer]

@ImportanceOfBeingErnest you 👍'd this, do you want to review it? :) or @WeatherGod?

[anntzer]

rebased

[QuLogic]

Seems to have really broken tests.

[anntzer]

likely because this crossed wires with the 3d projection change -- regenerated the baseline.