This class represents a subspace (component) of a multimesh function space.
The subspace is specified by an array of indices. For example, the array [3, 0, 2] specifies subspace 2 of subspace 0 of subspace 3.
A typical example is the function space W = V x P for Stokes. Here, V = W is the subspace for the velocity component and P = W is the subspace for the pressure component. Furthermore, W = V is the first component of the velocity space etc.
Create subspace for given component (one level)
MultiMeshSubSpace(V, component, sub_component)
Create subspace for given component (two levels)
Create subspace for given component (n levels)
The membership flag