Updating fitacfclientgui to add support for toggling between gate and slant range

[egthomas]

This pull request adds support to fitacfclientgui for toggling between the range gate number and slant range (km) along the top of the display by pressing the r key or with a new -r command line option. There has also been a small change to only print the header origin information if it is actually present in the real-time data stream.

[ksterne]

See if we can add a screenshot or some kind of example of what's going on.

[egthomas]

Here is a screenshot showing real-time Iceland East data with range gates running along the top of the display at 10 gate increments (default):

and here is the same display a few seconds later after pressing the r key to display slant range instead: