Discrepancy for `get_cell_map` between `UnstructuredGrid` and other structures.

Hello all,

Currently there are two (in my opinion conflictive) implementations of `get_cell_map`:

```
get_cell_map(g::UnstructuredGrid) = g.cell_map
```

and

```julia
function get_cell_map(trian::Grid)
cell_to_coords = get_cell_coordinates(trian)
cell_to_shapefuns = get_cell_shapefuns(trian)
lazy_map(linear_combination,cell_to_coords,cell_to_shapefuns)
end
```

This was changed in PR #553 , and makes `get_cell_map` return different things notably for `UnstructuredDiscreteModel` (returns `LinearCombinationField`) and `UnstructuredGrid` (returns `AffineMap`).

My question is: is there a specific reason behind this? Isn't the geometry (and therefore the cell mappings) of a `DiscreteModel` governed by it's underlying grid?

If not, should we change the implementation for `DiscreteModel`s to defer this method to it's underlying grid?

Thanks!