Split AD for Multifield

[zjwegert]

(WIP) DO NOT MERGE

Sister PR to Gridap#1136

TODO:

• Tests
• SkeletonTriangulation

Notes:

The below was fixed in #180
It appears there is a bug when trying to operate on CellFields. This should go in a separate issue.

using Gridap
using GridapDistributed
using PartitionedArrays

np = (2,2)
ranks = with_debug() do distribute
  distribute(LinearIndices((prod(np),)))
end

domain = (0,1,0,1)
partition = (5,5)
model = CartesianDiscreteModel(ranks,np,domain,partition)
Γ = BoundaryTriangulation(model)
V1 = FESpace(Γ,ReferenceFE(lagrangian,Float64,1))
V2 = FESpace(model,ReferenceFE(lagrangian,Float64,1))
uh1 = zero(V1);
uh2 = zero(V2);
uh1*uh2

This produces an error because _to_common_domain fails with the unreachable error. This appears to be because of the following: for DistributedTriangulations resulting from BoundaryTriangulation, we're in Case 4 of add_ghost_cells , so we create a new create a new triangulation with the ghost cells. The issue then appears to be that the glue information is missing, so both cases of

is_change_possible(_trian_a,_trian_b)
is_change_possible(_trian_b,_trian_a)

return false.