Add formatted KDI572/574 properties

Add formatted KDI572/574 properties

Syd Adams:
I've added an update to dme.cxx /dme.hxx.
It adds a formatted KDI572-574/nm , min and kt string properties meant
for text animations for these instruments.Existing instruments will funtion
as usual, no breaks. The purpose here is to eliminate yet another nasal
workaround which usually needs to repeat some of what the code already does.
The 3 properties are empty when the dme is off (no need to check for power),
dashes when powered up but no source , out of range ,etc.
The are formatted to the KDI-572/573/574 limits ...
0-99.9/100-389 for distance to station,
0-99 minutes for time to station,
groundspeed 0-999 kt.