Make template Calendar configurable in DateRangePrefixTree

DateRangePrefixTree (a SpatialPrefixTree designed for dates and date ranges) currently uses a hard-coded Calendar template for making new instances. This ought to be configurable so that, for example, the Gregorian change date can be configured. This is particularly important for compatibility with Java 8's java.time API which uses the Gregorian calendar for all time (there is no use of Julian prior to 1582).