[!16] Resolve "Documentation debt"

Merge branch '18-documentation-debt' into 'master'

ref:copasi/dune-copasi

### What does this MR do?

Adds more information to the README, and documents all the C++ classes and its
methods.

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

<!-- Is there something a reviewer should look out for _especially_? -->

I don't think so

### Can this MR be accepted?

-   [x] Implemented
    -   [x] Updated README
    -   [x] C++ classes documented
-   [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 [#18]

<!-- For automatic closing, do not forget the commas between issue numbers-->

<!--
PLEASE READ THIS!

A Merge Request should be associated to a certain task or issue.
Its changes are supposed to be merged into the master branch.

Briefly explain __how__ you achieved the proposal of the task.

IMPORTANT: Make sure to set the merge request WIP if you are not finished yet.
-->

See merge request [!16]

  [Build Status]: https://gitlab.dune-project.org/copasi/dune-copasi/badges/18-documentation-debt/pipeline.svg
  [![Build Status]]: https://gitlab.dune-project.org/copasi/dune-copasi/pipelines
  [1]: https://travis-ci.org/SoilRos/dune-copasi.svg?branch=18-documentation-debt
  [![Build Status][1]]: https://travis-ci.org/SoilRos/dune-copasi/branches
  [2]: https://ci.appveyor.com/api/projects/status/6605joy2w17qvca8/branch/18-documentation-debt?svg=true
  [![Build status][2]]: https://ci.appveyor.com/project/SoilRos/dune-copasi/history
  [#18]: gitlab.dune-project.org/NoneNone/issues/18
  [!16]: gitlab.dune-project.org/copasi/dune-copasi/merge_requests/16


Closes #18