Added OneToMany Relationship between Switch - Port entities
Modified SwitchDataCollector to get Ports from Switch entity
Added Testcase to MeasurementBackendTest
Added methods in Main for starting and stopping the collecting of measurements
Added method in Main for adding a switch to database
Modified SwitchDataCollector in Main to have one MeasurementDatabase object per thread, as implementation of a thread safe singleton MeasurementDatabase had implications on the transactions.