Code refine:
  1) Code style refine, add comment in DBFilter.h
  2) Refine the DBFilter.cpp.
      a) Avoid openTable/OriginDB everywhere in DBFilter.cpp by initialize the OriginDB and ResultTables in the
         constructor of DBFilter;
      b) Since the DBFilter keeps a DBName, so remove the in-arg 'aDBName' in many mem-func(s)

For now, a instance of DBFilter is created everytime filtering a origin-database, and released after the filtering.