Fix for out of memory error due to debug routine called 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().