Add get_axis_by_index function

[gbrammer]

This PR adds a function to return the axis from fig.axes corresponding to a particular pair of plotted data.

For example

import numpy as np
import corner

np.random.seed(1)
data = np.random.normal(size=(1000, 3))

labels = ["a", "b", "c"]
fig = corner.corner(data, labels=labels)

# The axis with x=a, y=b
ix = labels.index("a")
iy = labels.index("b")
ax = corner.axis_from_param_indices(fig, ix, iy)

ax.text(
    0.5, 0.5,
    f"x={labels[ix]}   y={labels[iy]}",
    fontsize=14,
    ha="center", va="center",
    color="r", transform=ax.transAxes, bbox={"fc": "w"}
)

# Test the inverse: ix = iy = 0 is the histogram axis for "a"
ix = iy = 0
i = corner.axis_from_param_indices(fig, ix, iy, return_axis=False)
ix_i, iy_i = corner.param_indices_from_axis(fig, i)
ax = fig.axes[i]

ax.text(
    0.5, 0.5,
    f"x={labels[ix_i]}  (histogram)",
    fontsize=14,
    ha="center", va="center",
    color="r", transform=ax.transAxes, bbox={"fc": "w"}
)

[dfm]

Thank you!