Update README of ``DAE`` project

[lisawim]

Since more files were added to the DAE project README.rst got an update by adding the new files to the list there. Further, I added a section how to implement a DAE problem in pySDC.

[brownbaerchen]

Are you sure the implementation details are set in stone at this point? You can consider adding the tutorial as documentation to the actual code of the class you implement here and then add the code to the readme. This is done in the other tutorials and makes sure the tutorials stay up to date when you change the code. I do appreciate your dedication to documentation!

[lisawim]

Thanks a lot for your idea! I did catch what you suggested but I'm not quite sure yet how to realise that.

[brownbaerchen]

Don't worry, it's not difficult. I did it recently in #417. You can use .. literalinclude:: to include a file of code or any txt file or whatever into the Readme. You can run the sphinx command locally to see the finished website, with the paths as they are in the online version. It can get a bit tricky with the relative paths...

[lisawim]

In order to follow the suggestion of @brownbaerchen I moved problem classes from simple_DAE.py into separate files so that each file only contains one problem class which is cleaner. First I wanted to use .. literalinclude:: for only specific lines but this is quite non-flexible, since lines could change and then for the README they need to be changed as well (which could be forgotten). Creating files also requires adaption of the files in run and the test files. Further, I (tried to add) a .. literalinclude:: in the README but it seems to not work yet. I guess I'm not sure which path I need to add. I thought the current one is correct, but this seems to be not the case.. Help is appreciated!

[lisawim]

This probably works now..

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.75000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 78.26%. Comparing base (18989d3) to head (8d9f75d). Report is 17 commits behind head on master.

Files	Patch %	Lines
pySDC/projects/DAE/problems/pendulum2D.py	88.00%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
+ Coverage   77.38%   78.26%   +0.87%     
==========================================
  Files         327      334       +7     
  Lines       26085    26303     +218     
==========================================
+ Hits        20187    20585     +398     
+ Misses       5898     5718     -180     

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[pancetta]

Famous last words here? @brownbaerchen ? Anyone?

[lisawim]

Not from my side.

[brownbaerchen]

Nope, let's go! :)