Unit Converter Dialog: Special support for DateTime/Numeric value conversion
Bug Number: n/a
Release notes (y/n): n
For Release Nums: 7.1

The Unit Converter dialog, for converting a single scaled-unit numeric value 
into different units of the unit type now has special support when the 
DateTime unit type is selected. In that case, different input and result 
widgets are shown. This new DateTime provision has two modes (selected with 
radio buttons):

  (1) DateTime to Numeric Value
  (2) Numeric Value to DateTime

This supports not only fully specified absolute date/times, but also 
"Partial" (periodic) DateTimes (e.g. "Month and Day"). These are different 
"units" within the "DateTime" unit type.  Note that the whole-number part of 
numeric (double precision float) values is the number of seconds since the 
beginning of the year 1800.  Absolute date/times don't have a fractional part.
Partial DateTimes do have a fractional part which identifies the significant 
time "parts".  (Up to six binary fractional digits are used, which does also 
work out to six decimal fractional digits).

See these six screenshots of the revised Unit Converter dialog:

http://cadswes2.colorado.edu/~philw/2017/Units/DateTime/UnitConvDlg/2017-06-06/01-UnitConv-Flow.png
http://cadswes2.colorado.edu/~philw/2017/Units/DateTime/UnitConvDlg/2017-06-06/02-UnitConv-UnitTypeMenu.png
http://cadswes2.colorado.edu/~philw/2017/Units/DateTime/UnitConvDlg/2017-06-06/03-UnitConv-DTime-to-Num1.png
http://cadswes2.colorado.edu/~philw/2017/Units/DateTime/UnitConvDlg/2017-06-06/04-UnitConv-DTime-to-Num2.png
http://cadswes2.colorado.edu/~philw/2017/Units/DateTime/UnitConvDlg/2017-06-06/05-UnitConv-DTime-to-Num-Partial.png
http://cadswes2.colorado.edu/~philw/2017/Units/DateTime/UnitConvDlg/2017-06-06/06-UnitConv-Num-to-DTime.png

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