Slate codegen improvement

[sv2518]

We should optimise code generation for transposes on Slate tensors by inlining the reversed indices into the local assembly kernel.

Lawrence words:

Right now, a Tensor(form) is translated by the slate compiler into something "opaque" I think. This is because compile_form spits back a loopy kernel object.

But think about what Transpose(Tensor(form)) does, it calls the generated loopy kernel to populate a tensor A (say), and then makes more gem that is CT(A[i, j], (j, i)).

If the gem that creates A were still around, we could inline that indexing transpose right into the kernel from tsfc. This would be morally equivalent to turning Transpose(Tensor(form)) into compile_form(form, transpose=True) (if compile_form had a transpose operation).

Perhaps this is already done because Transpose(Tensor(form)) can be turned into Tensor(adjoint(form)) and maybe you do this right now?

So what this is saying is that rather than:

1) gem_expr_for_form <- TSFC generates GEM from ufl form associated with Slate tensor
2) loopy_kernel_for_form <- TSFC generate loopy from gem_expr_for_form
3) gem_expr_for_slateops <- Slate compiler generates GEM for Slate operations
4) loopy_kernel_for_slateops <- TSFC generates loopy from gem_expr_for_slateops

where DiagonalTensors and Transposes are dealt with in step 3, we want to do

1) gem_expr_for_form <- TSFC generates GEM also from all modified terminals (Diagonals and Transposes e.g.)
2) loopy_kernel_for_form <- TSFC generate loopy from gem_expr_for_form
3) gem_expr_for_slateops <- Slate compiler generates GEM for Slate operations
4) loopy_kernel_for_slateops <- TSFC generates loopy from gem_expr_for_slateops

where DiagonalTensors and Transposes and so forth are dealt with in step 1 and step 3) and 4) only apply if there are more Slate operations on top of transposes.