optionals getAs to orElseThrow

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

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

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

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

...va/gov/usgs/earthquake/nshmp/data/DelimitedDataTests.java

.../java/gov/usgs/earthquake/nshmp/geo/json/GeoJsonTest.java

...a/gov/usgs/earthquake/nshmp/geo/json/PropertiesTests.java