Complete documentation

[rs028]

• Add LaTeX files (including figures) of the manual to the doc/ folder, so that it can be updated.
• Add a CHANGELOG file to the main directory summarizing the changes since version 7.4.1 (I think this was the starting version of @JosueBock development work)
• Add a CREDITS file to the main directory with the names of everybody who has contributed code over the years (this may require some digging, so let's start with the current list of names).
• Any outstanding issues and things to do/check/fix/improve (and other things that need to be kept in mind for future development) should be listed as open issue on the github issues page.
• There needs to be a little more explanation on how to use the model than the few lines in the README file.

[pb866]

Also, developers should be able to contribute to the wiki, so we can document our changes there.
Ideally, bit by bit the pdf manual should be translated to md files in the wiki. The repository should not have binary files to keep it clean and the size reasonable. If needed at all, pdf manuals should only be attached to releases in the release page.

[rs028]

I see now there is a changelog.txt in src/. It doesn't seem to record all the changes, though.
It also includes 2 items as "TO DO", which should be added as issues on github with a little more explanation.

[JosueBock]

Regarding the changelog and credits, I did this work directly in the code.
However, I agree that a summary of both could be provided in the documentation.

Regarding the instructions to use the model, I also agree that it has to be improved.