Added a builder for line-of-sight lists.

src/main/java/org/orekit/rugged/los/FixedLOS.java

src/main/java/org/orekit/rugged/los/LOSBuilder.java

src/site/markdown/design/overview.md

src/site/markdown/design/technical-choices.md

src/site/xdoc/changes.xml

src/test/java/org/orekit/rugged/api/RuggedBuilderTest.java

src/test/java/org/orekit/rugged/api/RuggedTest.java

...orekit/rugged/linesensor/SensorMeanPlaneCrossingTest.java