More work on raster coverage elevation dimension default value.:
WMS getDefaultElevation method - revised to throw ServiceException if
default value is invalid instead of returning the smallest numeric value.
WMSTest: add tests for getDefaultElevation method.
DimensionEditor: revise to use html select or textBox depending on how may
options are present.
ReaderDimensionsAccessor: revise scope of Comparator so can use it in WMSTest.