create_boundary....set is broken for mixed grids

[AbdAlazezAhmed]

Ran into it while testing #1108 , probably

Ferrite.jl/src/Grid/utils.jl

Line 168 in a38ed02

length(facets(cell)) < facet_nr && continue

will solve it

MWE:

 using Ferrite

nodes = [Node((-1.0, 0.0)), Node((0.0, 0.0)), Node((1.0, 0.0)), Node((-1.0, 1.0)), Node((0.0, 1.0))];

cells = [
                   Quadrilateral((1, 2, 5, 4)),
                   Triangle((3, 5, 2)),
               ];

grid = Grid(cells, nodes);

topology = ExclusiveTopology(grid);

 filter_function(x) = x[1] > 0;

addboundaryvertexset!(grid, topology, "test_boundary_vertexset", filter_function);

ERROR: BoundsError: attempt to access Tuple{Tuple{Int64, Int64}, Tuple{Int64, Int64}, Tuple{Int64, Int64}} at index [4]
Stacktrace:
 [1] getindex(t::Tuple, i::Int64)
   @ Base ./tuple.jl:31
 [2] (::Ferrite.var"#_makeset#174"{…})(ff_nh::Ferrite.CollectionsOfViews.ArrayOfVectorViews{…})
   @ Ferrite ~/.julia/dev/Ferrite/src/Grid/utils.jl:168
 [3] _create_boundaryset(f::typeof(filter_function), grid::Grid{…}, top::ExclusiveTopology, ::Type{…}; all::Bool)
   @ Ferrite ~/.julia/dev/Ferrite/src/Grid/utils.jl:182
 [4] _create_boundaryset(f::Function, grid::Grid{…}, top::ExclusiveTopology, ::Type{…})
   @ Ferrite ~/.julia/dev/Ferrite/src/Grid/utils.jl:158
 [5] create_boundaryvertexset
   @ ~/.julia/dev/Ferrite/src/Grid/utils.jl:210 [inlined]
 [6] addboundaryvertexset!(grid::Grid{…}, top::ExclusiveTopology, name::String, f::Function; kwargs::@Kwargs{})
   @ Ferrite ~/.julia/dev/Ferrite/src/Grid/utils.jl:100
 [7] addboundaryvertexset!(grid::Grid{2, Ferrite.AbstractCell, Float64}, top::ExclusiveTopology, name::String, f::Function)
   @ Ferrite ~/.julia/dev/Ferrite/src/Grid/utils.jl:99
 [8] top-level scope
   @ REPL[18]:1
Some type information was truncated. Use `show(err)` to see complete types.

[KnutAM]

That change looks good to me. Adding a comment why it works would be good + some test.

Not ideal that looping over the neighbourhood includes facet indices that the cell doesn't have, but avoiding this would require another layer outside to modify the iterations / allowed indices.