Name inconsistency for the vector of spaces between MultiFieldFESpace and DistributedMultiFieldFESpace

Hi @amartinhuertas , @fverdugo,

In Gridap, the member variable that contains the vector of `SingleFieldFESpace` in the `MultiFieldFESpace` is called `spaces`, while in `DistributedMultiFieldFESpace` is called `field_fe_space`.

In `GridapODEs` we are accessing this variable as `U.spaces`, which results in an error when `U` is a `DistributedMultiFieldFESpace`. Do you thing the name can be changed to match the definition of `MultiFieldFESpace`? Otherwise, we'll have to define a getter for both types:
```julia
get_spaces(U::MultiFieldFESpace) = U.spaces
get_spaces(U::DistributedMultiFieldFESpace) = U.field_fe_space
```

What would be the best solution?