Refactor the TernaryAxis code

[asinghvi17]

WIP. This aims to build the foundations for a ternary-axis layoutable, which can be directly plotted into.

The PR also has some other minor changes which I needed for my work.

There are 2 potential approaches to implementing this kind of thing:

1. Plot everything in a true 3D scene, and set the camera such that what you can see is the x + y + z = 1 plane. Then, draw the axis around that.
2. Extend what is done now, such that there is some nonlinear transformation which turns Point3f(x, y, z) into Point2f(u, v) (this is TernaryDiagrams.from_cart_to_bary). Then, following the method of GeoAxis, change the transform of the ternary axis' plots to this transform.

Both approaches will be somewhat technically challenging. Would appreciate any help!

[stelmo]

This is great! Thanks for the interest. I don't have time to make major changes now, but after mid-March my schedule opens up quite a bit. It is on my list :)

[stelmo]

But feel free to continue this PR, I will catch up with you next month!

[asinghvi17]

Sounds good! This will be pretty slow burn in any case, as and when I get time for it :)

[asinghvi17]

OK, so I hacked a bit on this and figured I should change the code first before doing anything else. This relies on Makie master for now but is otherwise just a refactoring of the ternary axis recipe.

It brings the ternary axis theme in line with the rest of Makie's axis themes, for later integration as a layoutable (Block). Here is a (brief) list of changes I've made:

1. Arrows are now drawn by the new bracket! recipe, which maintains the correct rotation at all times.
2. The theme has been changed to reflect the theme of e.g. Axis. As such, labelx -> xlabel, and all attributes are per individual label, not global to all labels in a TernaryAxis.
3. The ticks have been changed - the grid and minor grid are now plotted separately, and may have separate colors as a result. These can be themed easily.

TODOs:

• Get tick spacing/padding working correctly
• Add tick marks
• Get the spacing between the arrow and the axis to depend on the ticks' size + some scale factor
• Let the bracket arrow be themable