[!294] fix identity handling

Merge branch 'feature/fix-identity-handling' into 'master'

ref:extensions/dune-codegen Before, using this

    A = as_matrix([[x[i]*x[j] for j in range(2)] for i in range(2)]) + I

in a ufl file didn't compile, but

    A = as_matrix([[x[i]*x[j] + I[i,j] for j in range(2)] for i in range(2)])

did.

See merge request [extensions/dune-codegen!294]

  [extensions/dune-codegen!294]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/294