Configurable time units when using dateFormatMark

[tdrwenski]

When using a dateFormatMark with a joinExisting aggregation, the time axis values will be ISO strings such as "2000-01-01T00:00:00Z". See for example here.

However, it may be desirable to have numeric time values with udunits such as "seconds since 1981-01-01 00:00:00." Add a NcML option to allow this to be configurable.

See also: https://www.unidata.ucar.edu/mailing_lists/archives/thredds/2010/msg00413.html