St nicholas warnings fix day

- Fixed warnings concerning unused variables.
- Fixed warning related to the sequence of attributes in initializer list.
- Fixed warning "comparison between signed and unsigned".
- Fixing warning "unused parameter".

Still not fixed:
- Warnings concerning unused functions, since we will use these functions later on.
- Warnings in FileIO::RapidVtuInterface, since this class is ot used anymore and will be removed soon?!
- Warning in BoostVtuInterface, since in future we want to use vtk-6 functionality for this purpose.