No documentation on how to access the 2D surface returned by MembThickness

[p-j-smith]

Which section of the documentation needs improving?
Please provide a link (if it is a specific page).

https://lipyphilic.readthedocs.io/en/latest/reference/analysis/memb_thickness.html#lipyphilic.analysis.memb_thickness.MembThickness

What can be improved about this section
Is it incomplete, incorrect or difficult to understand?

As pointed out int #142, there is no documentation on how to access the 2D surface returned by MembThickness if return_surface is true.

How to improve this section
Do you have any specific suggestions we could take to improve the documentation?

The surface is stored in the attribute memb_thickness_grid:

lipyphilic/src/lipyphilic/analysis/memb_thickness.py

Lines 225 to 229 in 3d9342e

	if self._return_surface:
	self.memb_thickness_grid = np.full(
	(self.n_frames, self.n_bins, self.n_bins),
	fill_value=np.NaN,
	)

This should be added to the description of the return_surface argument.