Issue 2521 emit warning

[fluidnumerics-joe]

This PR adds a helper function to spatialhash.py to create a mask for degenerate cells. In the spatialhash initialization, this method is called for the case when the grid is an xgrid object with spherical mesh type. In the event any degenerate faces are found, a warning is emitted. This is related to the action items in #2521

[erikvansebille]

Looks good to me. Just one suggestion below

[VeckoTheGecko]

Looks good from a code POV

@fluidnumerics-joe am i right to say that all incomplete meshes (i.e., where there are 0s in for lon and lat) will have these incomplete "genereate" cells and hence raise this warning?

Just good to know one way or another - we can't really detect whether these cells are in coastal regions or not anyway so there's a limit to what we can do

[fluidnumerics-joe]

Looks good from a code POV

@fluidnumerics-joe am i right to say that all incomplete meshes (i.e., where there are 0s in for lon and lat) will have these incomplete "genereate" cells and hence raise this warning?

Just good to know one way or another - we can't really detect whether these cells are in coastal regions or not anyway so there's a limit to what we can do

In #2521 , it was indeed the case that the location of lat,lon = 0 locations corresponded to land cells and this mesh was "incomplete". these incomplete meshes will definitely have degenerate cells. In general, any face that is abnormally large in comparison to others will cause problems. Remember that we're constructing faces over the stencil [i:i+1,j:j+1] for lat and lon; if that face ends up spanning the entire grid, that will likely get picked up as a degenerate cell.

The best we could do, is filter out the degenerate cells in the hash table construction. This would prevent them from ever being used in a search and would guard against unnecessary large memory allocations.

After throwing the warning, I'm wondering if we would want to go ahead and filter these out and then construct the hash table. In this case, if a particle ever landed in a degenerate face, the spatial hashing search would just fail to find it.

Right now, we throw a warning and then an oom error will be thrown

[VeckoTheGecko]

Gotcha, thanks for the clarification @fluidnumerics-joe !