Use of set to test uniqueness of grids in wrap_func_objects results in unwanted behavior.

[JoerivanEngelen]

To create a list of unique grids, set is used

xugrid/xugrid/core/common.py

Line 101 in e74fbc1

grids = set(grids)

However, this tests if grids refer to the same object, which results in behaviour like this:

import xugrid as xu

a = xu.data.disk()
grids = xu.concat([a, a], dim="foo").grids
print(len(grids)) # prints 1

grids2= xu.concat([xu.data.disk(), xu.data.disk()], dim="foo").grids
print(len(grids)) # prints 2

We want to test if grids are equal, as to be fixed in this issue:
#205

[Huite]

Adding to some of the things in #205
If objects are equal, but not exactly (i.e. one has a celltree initialized, but the other doesn't), in general you'd want the richer object. But there's no clear definition of that, so I'd say: just take the first if there's many.