Refactoring Grand Prix managment

 * Formerly, there were setter methods in grand_prix_data.cpp which destroyed the concept of privat and public for these members. I've replaced those with ```friend``` and direct acces.
 * A check detects now negativ lap numbers in Grands Prix which otherwise lead to a sigsegv.
 * There are many additional comments identation fixes, clean-ups and lines longer than 80 characters have been shortend. e.g. a ```throw(std::logic_error)``` after a method declaration and some wrong wrong ```const``` were removed. 
 * ```GrandPrixData::reload()``` has now a totally new structur which fits better to the xml files and is much clearer and better commented.
 * I've got some corrupt .grandprix files to test if all the tests in ```GrandPrixData::reload()``` work , but I don't know where to put them.

I'm planning to correct the [spelling of Grand Prix](http://en.wiktionary.org/wiki/Grand_Prix) and to do some renaming, moving and refactoring, so it might be better not to merge this before I've implemented that, but I'ld like to get some feedback on my already finished work.