Merge branch 'feature/check-for-submodule-existence' into 'master'

See merge request [extensions/dune-codegen!298]

  [extensions/dune-codegen!298]: Noneextensions/dune-codegen/merge_requests/298

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

Merge branch 'feature/geometry-mixins' into 'master'

ref:extensions/dune-codegen This MR switches the way we allow configurable
handling of geometry, quadrature and basis evaluation. So far, we had an
interface class - one for PDELab, sum factorization and block structuring.
This had the disadvantage of being quite monolithic. Also, different
geometries (equidistant, axiparallel, multilinear etc.) were handled through a
backend mechanism that performed fine for simple examples, but could get quite
messy for complex ones.

The new way introduced here is mixin classes. Mixins for the above topics can
be defined anywhere in the code (even in user code, if a poweruser wants to
implement his own geometries!!!) and are registered through a decorator. The
visitor is then constructed by choosing a set of mixins and combining them
into a class. This is currently done through ini options.

TODO:

-   \[x\] Allow generators to be used as class methods
-   \[x\] Implement basic mixin infrastructure
-   \[x\] Construct a visitor by combining mixins
-   \[x\] Port existing geometry implementations
    -   \[x\] Generic PDELab
    -   \[x\] Axiparallel
    -   \[x\] Equidistant
    -   \[x\] Blockstructured
    -   \[x\] Sumfact: Multilinear
-   \[x\] Remove *Interface* Concept in favor of mixins
-   \[x\] Final cleanup

This fixes [#128].

See merge request [extensions/dune-codegen!287]

  [#128]: gitlab.dune-project.org/NoneNone/issues/128
  [extensions/dune-codegen!287]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/287


Closes #128

Merge branch 'feature/vetorization-unstructured-grids-cherry-pick' into 'master'

ref:extensions/dune-codegen

-   Make it possible to do vectorization on unstructured grids
-   Move most things concerning permutations into the interface classes of the
    sumfact kernel
-   Add test cases for all variations of facedir and facemod

See merge request [extensions/dune-codegen!285]

  [extensions/dune-codegen!285]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/285

The realize_output interface method was not really necessary and just adds a
layer of indiretion.

Due to a bug in dune-geomtry the volume calculation is not accurate enough. For
now we just remove all volume calls from the weak formulation.

Merge branch 'feature/geometry-mixins' of ssh://gitlab.dune-project.org:22022/extensions/dune-codegen into feature/geometry-mixins