Fix find_all_active_cells_around_point()

[peterrum]

@mschreter Could you take over and add a test. Thx!

[mschreter]

This PR extends find_all_active_cells_around_point() to return the right number of cells for a point located on the cell edge/corner between elements of different refinement levels, e.g. as the pink marked point in the figure below.

Previously if the pink point was requested within RemotePointEvaluation, the following assert would have been triggered:

--------------------------------------------------------
An error occurred in line <352> of file </home/much/work/libs/dealii/include/deal.II/grid/tria_iterator.templates.h> in function
    dealii::TriaActiveIterator<Accessor>::TriaActiveIterator(const dealii::TriaIterator<Accessor>&) [with Accessor = dealii::CellAccessor<1, 1>]
The violated condition was: 
    this->accessor.has_children() == false
Additional information: 
    (none)

@nmuch FYI

[mschreter]

Ready to review from my side!

[peterrum]

/rebuild

[peterrum]

@mschreter Could you add a change-log entry?

[peterrum]

@mschreter Thanks 👍

[peterrum]

@mschreter When looking at the picture in #14728 (comment), I think you are testing at the wrong place. It should be at (0.5, 0.25). Also would it be possible to switch the order of refinement. You have refined left. I would like to see also on the right. The motivation is that it makes the difference from which side the algorithm is run.

[mschreter]

@mschreter When looking at the picture in #14728 (comment), I think you are testing at the wrong place. It should be at (0.5, 0.25). Also would it be possible to switch the order of refinement. You have refined left. I would like to see also on the right. The motivation is that it makes the difference from which side the algorithm is run.

I've tested (0.25,0.25), which is not a hanging node but is connected to cells at different levels. I thought this should be a critical point as well (not only hanging nodes). Nevertheless, I could extend the test case in a follow-up PR regarding (i) including a hanging node request and (ii) switching refinement direction.