Apply new programming style guidelines

I applied the new style guidelines that can now be found in the wiki. The most important changes are:

• real_t is changed to rational
• a lot of const and constexpr keywords were added
• all names (variables, types, files, ...) are now in accordance with the guidelines
• local variables are now defined in the smallest scope possible
• basic indent is now 2 instead of 4
• headers, #include statements and comments got a rework
• some changes were suggested by cppcheck, which I recommend to use

The code beautifier Uncrustify is used to help enforce the style guidelines. It should be executed before commiting any changes.