combine `ax1` and `ax2` into `ax` for `mpu.colorbar`

[mathause]

• mpu.colorbar currently takes ax1 and ax2 as input. https://github.com/mathause/mplotutils/blob/e48e8d156990f269b786e0d7f29b3c8c3ac2f16f/mplotutils/_colorbar.py#L10-L13
• plt.colorbar, in contrast uses

  plt.colorbar(
     mappable: 'ScalarMappable | None' = None,
     cax: 'matplotlib.axes.Axes | None' = None,
     ax: 'matplotlib.axes.Axes | Iterable[matplotlib.axes.Axes] | None' = None,
     **kwargs,
  ) -> 'Colorbar'

We should also set

 def colorbar( 
     mappable, 
     ax: 'matplotlib.axes.Axes | Iterable[matplotlib.axes.Axes],
     *,
     ...
)

(deprecating ax1 and ax2) so that both take ax as argument.

[mathause]

This would also allow to pass all axes and then use the rightmost/ bottommost of them.