Merge branch '20-add-special-membranes-for-species-that-only-pass-through-to-it-but-not-diffuse' into 'master'

ref:copasi/dune-copasi

### What does this MR do?

-   [x] Add a Finite Volume (FV) local operator.
-   [x] Re-structure the pdelab tree to allow both, Finite Volume as well as
    Conforming Finite Elements.
    -   [x] Make stand-alone diffusion-reaction model for CG work again
    -   [x] Make a stand-alone diffusion-reaction model for FV
        -   [x] Add similar test to what is already in multidomain (or improve
            them e.g. test agains analytic functions with an L2 error
            integration)
        -   [x] Write cell data by dafault
    -   [x] Make a multidomain diffusion reaction model with only FV
    -   [ ] ~~Manage domains with no variables by mapping domains to gfs index
        (yet another level of indirection)~~
    -   [ ] ~~Make a stand-alone diffusion-reaction model with composite FV
        and CG~~
    -   [ ] ~~Make a multidomain diffusion-reaction model with composite FV
        and CG~~
    -   [x] Use the virtual interface of local finite elements to choose
        between FV and CG discretization
-   [x] Allow selective refinement per compartment i.e. anisotropic cube
    refinement.

### Is there something that needs to be double checked?

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### Can this MR be accepted?

-   [x] Implemented
-   [x] Added/Updated tests:
-   [x] Added/Updated documentation
-   [x] Pipelines passing <!-- please check for new warnings -->
     <!-- change all occurences of <branch> for your branch name -->

    -   [x] [![Build Status]]
    -   [x] [![Build Status][1]]
    -   [x] [![Build status][2]]

-   [x] Delete branch option set <!-- unless there's a good reason -->
-   [x] Added entry to CHANGELOG.md

### Related issues

Closes [#20]

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See merge request [!17]

  [Build Status]: https://gitlab.dune-project.org/copasi/dune-copasi/badges/20-add-special-membranes-for-species-that-only-pass-through-to-it-but-not-diffuse/pipeline.svg
  [![Build Status]]: https://gitlab.dune-project.org/copasi/dune-copasi/pipelines
  [1]: https://travis-ci.org/SoilRos/dune-copasi.svg?branch=20-add-special-membranes-for-species-that-only-pass-through-to-it-but-not-diffuse
  [![Build Status][1]]: https://travis-ci.org/SoilRos/dune-copasi/branches
  [2]: https://ci.appveyor.com/api/projects/status/6605joy2w17qvca8/branch/20-add-special-membranes-for-species-that-only-pass-through-to-it-but-not-diffuse?svg=true
  [![Build status][2]]: https://ci.appveyor.com/project/SoilRos/dune-copasi/history
  [#20]: gitlab.dune-project.org/NoneNone/issues/20
  [!17]: gitlab.dune-project.org/copasi/dune-copasi/merge_requests/17


Closes #20

Merge branch '20-add-special-membranes-for-species-that-only-pass-through-to-it-but-not-diffuse' of ssh://gitlab.dune-project.org:22022/copasi/dune-copasi into 20-add-special-membranes-for-species-that-only-pass-through-to-it-but-not-diffuse

* Library modes can be switched by default in CMake with the option BUILD_SHARED_LIBS
  hence no need to hardcode them with different targets.
* Exported targets were not working properly because the config file didn't contain enough
  information (DUNE creates one by default). So I remove it and will refactor all CMakeFiles
  later to use modern CMake directly (without DUNE macros).

restore clone_folder, but create temp dir to download & extract static libs into to avoid overwriting dune-copasi cloned files

This reverts commit 7b297f22.