Fixes for compiler warnings

These patches fix some warnings during Kwant's build process, namely a comparison between an unsigned and a signed int in partitioner.cc, one maybe-uninitilized warning in operator.pyx and an incompatible-pointer-types warning in graph/util.pyx

These warnings are somewhat spurious, because the signed number will never be negative, the quantity in operator.pyx will always be initalized when used and the pointer types in graph/util.pyx are obviously compatible, but all of these are written in a way, so that GCC cannot reason about the Code sufficiently and emits the warnings anyway. I think it is good to squelch all the warnings, so this is what I did.