updates for grid support

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

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

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

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

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

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

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

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