Added DatesSelector for managing continuous streams or bursts of dates.

src/design/measurements-generation-class-diagram.puml

...estimation/measurements/generation/AbstractScheduler.java

...timation/measurements/generation/EventBasedScheduler.java

src/main/java/org/orekit/time/BurstSelector.java

src/main/java/org/orekit/time/DatesSelector.java

src/main/java/org/orekit/time/FixedStepSelector.java