add basic frame of program
add Package DAO, Class Datum, Tuple, Operator, ScanOperator
fixed and tested ScanOperator.java,
it can be used now.
add SelectionOperator, Evaluator
not tested, incomplete implementation
fully implemented selectionOperator,
not tested
complete projection
aggregate function : count, sum
aggregator
run through nba01,02,03,04
version 1, changed all the names of operator
version 1.1
1.11
1.2
1.21
1.22
1.3 nba4
1.31
1.4
1.5 finish orderby
1.51