Make Standard::Purge() working hard to return more memory to the system

Documentation remark, added by Author 2009-11-04 10:36:19:

Improvements:
Memory management.
The method Standard::Purge has been improved. The previous implementation of
this method in the class Standard_MMgrOpt released only free memory blocks of
sizes greater than the value of MMGT_CELLSIZE (200 by default). Smaller blocks
are allocated from within so named memory pools, and they were never returned to
the system. With this improvement, the method Purge() finds the pools that do
not contain any busy blocks and releases them. After that the virtual address
space is really returned to the system.