Compiler warnings

We should fix or at least discuss whether we want to fix the following compiler warnings (I am aware that DUNE prints a lot of this warnings, too).

• -Wshadow (shadowing of local variables, e.g. by lambda functions or if/for loops)
• -Wno-missing-braces -Wmissing-field-initializers (initiliazation, e.g. object({}) vs object{{}} )
• -Wunused-result (e.g. #472 (closed))
• -Wfloat-equal