Feature: get available slots of a certain size

close #5  
for a given PolicyAllocator of type T, you can check availability of the FreeMemory functionality through a trait:
```c++
bool available1 = PolicyMalloc::Traits<T>providesAvailableSlotsHost;
bool available2 = PolicyMalloc::Traits<T>providesAvailableSlotsAccelerator;

// on host:
if(available1){
  unsigned free1MB = PolicyMalloc::getAvailableSlotsHost(1*1024*1024);
}

// on accelerator:
if(available2){
  unsigned free1MB = PolicyMalloc::getAvailableSlotsAccelerator(1*1024*1024);
}
```