Add double parameter class

src/main/java/gov/usgs/earthquake/nshmp/netcdf/www/meta/DoubleParameter.java

+package gov.usgs.earthquake.nshmp.netcdf.www.meta;
+
+public final class DoubleParameter {
+
+  private final String name;
+  private final String units;
+  private final double min;
+  private final double max;
+
+  public DoubleParameter(String name, String units, double min, double max) {
+    this.name = name;
+    this.units = units;
+    this.min = min;
+    this.max = max;
+  }
+}