added variable time period

-added property activationTime (HashMap <String id ,Integer nextActivationTime>) to Simulation class. It collects times of next activation for each periodic task. 
-changed way getNextActivationTime() works. Now it finds min from activationTime
-updateReadyQ => updateReadyQandActivationTimes
-changed readInputFromFile
-PeriodicTask is not abstract class anymore. time period and execution time can now be variable.
-removed PeriodicTaskFixed, PeriodicTaskFrequencyTable, PeriodicTaskMinMaxUniform
-changed testCase input files to fit changes in algorithm