Define concept of `FunctionSpace` for mixed topology

[chrisrichardson]

When a mesh contains multiple cell types (e.g. triangle + quadrilateral), we need to be able to define a FunctionSpace on that mesh, so we can add Forms.
Currently, a FunctionSpace encapsulates: [Mesh, FiniteElement, DofMap]. I suggest we could change it to: [Mesh, [FiniteElements], [DofMaps]] where [FiniteElements] and [DofMaps] are lists which must be the same size as the number of cell types in the mesh.

[Digital365Staking]

Benefits of Your Suggestion:

Unified FunctionSpace:
The idea of having a FunctionSpace that can handle multiple element types by using lists for FiniteElements and DofMaps makes the system more flexible and generalizes the handling of mixed-topology meshes.
Instead of needing separate FunctionSpace objects for each cell type, the mesh itself can be fully represented within one object, making it easier to manage.

Consistency:
By having corresponding lists of FiniteElements and DofMaps, the system ensures that each element type on a particular cell corresponds to a specific DOF map, which helps maintain consistency and reduces errors when handling different cell types.

Simplified Interface:
Users can define a FunctionSpace on a mixed-topology mesh without worrying about creating separate spaces for each cell type. The system will handle the complexity internally, allowing users to work with a single FunctionSpace even when dealing with different element types (e.g., triangles and quadrilaterals).

Efficient Assembler:
The assembler can then loop over all FiniteElements and DofMaps in a single structure, processing each cell type accordingly. This avoids the need for multiple forms and allows the assembler to handle different kernels in a more efficient and unified way.

Maintainability:
With this approach, the data is centralized and consistent. It eliminates redundancy, which helps in maintaining and extending the codebase, especially as more cell types or element types are added.

Potential Challenges:

Handling Element Compatibility:
Different cell types (e.g., triangles, quadrilaterals) might require different computational treatment. Ensuring that the DofMap and FiniteElement for each cell type are compatible and correctly mapped might require additional care in the implementation.

Performance Considerations:
While this restructuring will likely simplify the code, performance optimization will still be important, especially if the assembler needs to loop over many different element types. Efficient memory access patterns and data storage will be crucial to avoid unnecessary overhead.

Complexity in Mesh-Element Mapping:
The FunctionSpace would need to properly map each cell in the mesh to its corresponding FiniteElement and DofMap. This mapping needs to be done carefully to ensure that each degree of freedom is correctly associated with its respective cell type.