mfd test docs and cleaning

src/main/java/gov/usgs/earthquake/nshmp/Maths.java

src/main/java/gov/usgs/earthquake/nshmp/mfd/Mfd.java

src/main/java/gov/usgs/earthquake/nshmp/mfd/Mfds.java

...ain/java/gov/usgs/earthquake/nshmp/model/Deserialize.java

...java/gov/usgs/earthquake/nshmp/model/FaultRuptureSet.java

src/main/java/gov/usgs/earthquake/nshmp/model/MfdTrees.java

.../usgs/earthquake/nshmp/mfd/IncrementalMfdBuilderTest.java

src/test/java/gov/usgs/earthquake/nshmp/mfd/MfdTests.java

src/test/java/gov/usgs/earthquake/nshmp/mfd/Mfds2Test.java

src/test/java/gov/usgs/earthquake/nshmp/mfd/MfdsTests.java