`FullGridCellList` can't handle `NaN` values and throws unspecific errors

See https://github.com/trixi-framework/PointNeighbors.jl/issues/64. ```julia julia> nhs = GridNeighborhoodSearch{2}(search_radius = 1.0, n_points = 2, cell_list = FullGridCellList(min_corner = (-10.0, -10.0), max_corner = (10.0, 10.0), search_radius = 1.0)); julia> initialize!(nhs, x, x); ERROR: BoundsError: attempt to access 22×22 LinearIndices{2, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}} at index [13, -9223372036854775808] Stacktrace: [1] throw_boundserror(A::LinearIndices{2, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}}, I::Tuple{Int64, Int64}) @ Base ./abstractarray.jl:737 [2] checkbounds @ ./abstractarray.jl:702 [inlined] [3] _getindex @ ./abstractarray.jl:1323 [inlined] [4] getindex @ ./abstractarray.jl:1291 [inlined] [5] cell_index @ /mnt/ssd/home/efaulhab/git/PointNeighbors.jl/src/cell_lists/full_grid.jl:162 [inlined] [6] push_cell! @ /mnt/ssd/home/efaulhab/git/PointNeighbors.jl/src/cell_lists/full_grid.jl:124 [inlined] [7] initialize_grid!(neighborhood_search::GridNeighborhoodSearch{…}, y::Matrix{…}) @ PointNeighbors /mnt/ssd/home/efaulhab/git/PointNeighbors.jl/src/nhs_grid.jl:176 [8] initialize!(neighborhood_search::GridNeighborhoodSearch{…}, x::Matrix{…}, y::Matrix{…}) @ PointNeighbors /mnt/ssd/home/efaulhab/git/PointNeighbors.jl/src/nhs_grid.jl:162 [9] top-level scope @ REPL[14]:1 Some type information was truncated. Use `show(err)` to see complete types. ``` We want to either make this cell list work with `NaN` values or at least throw an error message that properly explains the problem. The error message when points are outside the bounding box should also be more specific.