Addressed following TODO: 
//TODO: clarify when we use reportNumber, reportUnit.getReportNum() - 1
and reportUnit.getReportNum(). [Freek]
[Pravin] reportUnit.getReportNum() - 1 represents report index. Report
index is used for array operations. 
Added reportIndex variable and getReportIndex() method in ReportUnit to
avoid repetitive calls to reportUnit.getReportNum() - 1.
Replaced reportUnit.getReportNum() - 1 with reportUnit.getReportIndex()
Replaced reportNum with reportIndex in ChartUnit.java
Adjusted ChartUnitTest.java for using reportIndex