Enhance METviewer GUI to specify time formatting options.

[JohnHalleyGotway]

This request came in via MET-Help from Perry Shafran at NCEP:
https://rt.rap.ucar.edu/rt/Ticket/Display.html?id=87409

This issue includes 2 features:

(1) Perry would like to be able to specify the time formatting information. Suggest the following:

Enhance the time picker popup box by adding a new text box for "time format" which is by default, blank. The user can enter time formatting info like "%Y-%m-%d %H:%M:%S" and that formatting is used to populate the "Label" string. So the picker would parse these times, and if time formatting is specified, update the date/timestamps in the labels.

(2) On the "X1" tab, in the "X1 values formatting" box, the frequency entry seems to control the label plotting frequency. When plotting dates, the frequency seems to work when they are plotted vertically but does not work when they are plotted horizontally. Please debug the frequency when plotting horizontally.

Michelle Harrold also echoed that (1) would be a very nice feature in her work ask well.