applied sigma scaling in gm2curves

src/main/java/gov/usgs/earthquake/nshmp/calc/CalcConfig.java

.../java/gov/usgs/earthquake/nshmp/calc/ExceedanceModel.java

src/main/java/gov/usgs/earthquake/nshmp/calc/Transforms.java

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

src/main/resources/calc/calc-config-defaults.json

.../java/gov/usgs/earthquake/nshmp/calc/CalcConfigTests.java

src/test/resources/calc/calc-config-extends.json