Merge branch 'issue-536' into develop

src/changes/changes.xml

src/main/java/org/orekit/overview.html

...rekit/propagation/events/CircularFieldOfViewDetector.java

...va/org/orekit/propagation/events/FieldOfViewDetector.java

...java/org/orekit/propagation/events/VisibilityTrigger.java

src/site/markdown/index.md

...t/propagation/events/CircularFieldOfViewDetectorTest.java

...rg/orekit/propagation/events/FieldOfViewDetectorTest.java