Updated config to make the tests pass (dataDir should be used if it is specified
[Optimization] Only have a single VirtualTrackColumn for each column in a track. This is done to reduce the number of calls to get_table().
[Optimization] Cache nodes in the Database to reduce number of internal calls to get_nodes()
[Optimization] Only open when db is closed. To reduce number of internal calls to pytables' get_node()
[Optimization] Global state variable is used to determine whether preprocessor is run or not. To reduce number of callsto close