mfd restructuring and cleaning; logic tree implements list

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

...main/java/gov/usgs/earthquake/nshmp/mfd/package-info.java

...va/gov/usgs/earthquake/nshmp/model/ClusterRuptureSet.java

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

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

...main/java/gov/usgs/earthquake/nshmp/model/GridLoader.java

...n/java/gov/usgs/earthquake/nshmp/model/GridSourceSet.java

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

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

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

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

src/main/java/gov/usgs/earthquake/nshmp/tree/Branch.java

src/main/java/gov/usgs/earthquake/nshmp/tree/EnumBranch.java

.../java/gov/usgs/earthquake/nshmp/tree/EnumValueBranch.java

...main/java/gov/usgs/earthquake/nshmp/tree/GroupBranch.java

src/main/java/gov/usgs/earthquake/nshmp/tree/LogicGroup.java

src/main/java/gov/usgs/earthquake/nshmp/tree/LogicTree.java

...in/java/gov/usgs/earthquake/nshmp/tree/RegularBranch.java

...main/java/gov/usgs/earthquake/nshmp/tree/RegularTree.java

...ava/gov/usgs/earthquake/nshmp/tree/StringValueBranch.java