Fix warning sign compare

Fix a signed/unsigned warning in RestartParticleLoader

`localCellId` is unsigned (16 or 32 bit) while `PMacc::math::CT::volume<SuperCellSize>::type::value` is an `int`

Found on joker with gcc/4.6.2 and gcc/4.7.1.
pls let us use c++ style type casts :)
```c++
assert( uint32_t(localCellId) < uint32_t(PMacc::math::CT::volume<SuperCellSize>::type::value) );
```