Fix for out of memory error due to debug routine call during op_partition()

An out of memory error occurred during running a 0.2 Billion edges mesh on HECTOR (as part of a weak scaling study). The cause was identified to be due to diagnostics/debugging checks to verify that an `op_map` is an ONTO map. This check utilizes `MPI_Allgather` routines during `op_partition()` and as such also increases the time of performing the partitioning. 

This pull request moves this check to be a debug routine (enabled with `DEBUG`). Additionally the partition reversing call in `op_exit()` is also removed and is replaced with `op_partition_destroy()`. The reason for this removal is again to fix the out of memory error and also to reduce the time spent during `op_exit()`.