replace vector iterators with hand-written iterators

instead of relying on the properties of the vector iterators,
implement our own iterator that gracefully decays to a pointer.
This is save, since the C++ standard requires the vector to store
its elements in a continuous block of memory.

No more stupid Microsoft runtime checks.

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@2466 5a81b35b-ba03-0410-adc8-b2c5c5119f08