Add time dimension to data for display control

In getDataInner, repackage the FlatField into a FieldImpl with a
time domain.

This allows the Image display control to automatically
set a timestamp in the time animation widget, and properly fill in
the %timestamp% macro.

Also, handle the FieldImpl when creating the Field Selector preview
image.

[1429]
Merge pull request #89 from mhiley/inq1429

Add time dimension to data for display control