FullGridCellList can't handle NaN values and throws unspecific errors

[efaulhaber]

See #64.

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