Move AxisArtistHelpers to toplevel.

The axisartist has a concept of "axis_artist_helper", which computes
various computations to draw slanted/curved axises.  Confusingly,
`AxisArtistHelper` (and likewise `AxisArtistHelperRectlinear`) do *not*
define such helper classes; they are simply namespaces that hold the
`{AxisArtistHelper,AxisArtistHelperRectlinear}.{Fixed,Floating}` nested
classes which *do* define helpers.  More specifically,
`AxisArtistHelper.{Fixed,Floating}` act as abstract base classes for
`AxisArtistHelperRectlinear.{Fixed,Floating}` which are actually usable.

In order to slightly disentangle this move the actual helper classes to
the toplevel (as `_{Fixed,Floating}AxisArtistHelperBase` and
`_{Fixed,Floating}AxisArtistHelperRectlinear`), keeping the old
"purely namespace" classes around for backcompat.  (But note that end
users should never have to directly interact with these classes anyways
-- normally, they only construct GridHelpers which take care of the
interaction with AxisArtistHelpers; see e.g. the various axisartist
examples.)

More simply, this commit simply dedents most of the definitions of the
Helper classes.

Author: anntzer

doc/missing-references.json

@@ -277,8 +277,15 @@
     "mpl_toolkits.axisartist.axisline_style._FancyAxislineStyle.SimpleArrow": [
       "lib/mpl_toolkits/axisartist/axisline_style.py:docstring of mpl_toolkits.axisartist.axisline_style.AxislineStyle:1"
     ],
-    "mpl_toolkits.axisartist.axislines.AxisArtistHelper._Base": [
-      "lib/mpl_toolkits/axisartist/axislines.py:docstring of mpl_toolkits.axisartist.axislines.AxisArtistHelper:1"
+    "mpl_toolkits.axisartist.axislines._FixedAxisArtistHelperBase": [
+      "lib/mpl_toolkits/axisartist/axislines.py:docstring of mpl_toolkits.axisartist.axislines.AxisArtistHelper:1",
+      "lib/mpl_toolkits/axisartist/axislines.py:docstring of mpl_toolkits.axisartist.axislines.FixedAxisArtistHelperRectilinear:1",
+      "lib/mpl_toolkits/axisartist/grid_helper_curvelinear.py:docstring of mpl_toolkits.axisartist.grid_helper_curvelinear.FixedAxisArtistHelper:1"
+    ],
+    "mpl_toolkits.axisartist.axislines._FloatingAxisArtistHelperBase": [
+      "lib/mpl_toolkits/axisartist/axislines.py:docstring of mpl_toolkits.axisartist.axislines.AxisArtistHelper:1",
+      "lib/mpl_toolkits/axisartist/axislines.py:docstring of mpl_toolkits.axisartist.axislines.FloatingAxisArtistHelperRectilinear:1",
+      "lib/mpl_toolkits/axisartist/grid_helper_curvelinear.py:docstring of mpl_toolkits.axisartist.grid_helper_curvelinear.FloatingAxisArtistHelper:1"
     ],
     "mpl_toolkits.axisartist.floating_axes.FloatingAxesHostAxes": [
       "doc/api/_as_gen/mpl_toolkits.axisartist.floating_axes.rst:32:<autosummary>:1",