Visualization of data from moving platform radars which are not geo-references

[jjhelmus]

A method for visualizing data from moving platforms which do not contain geo-reference data should be added to Py-ART. Data from these platforms have latitude, longitude and altitude variables which vary in time but do not have geo-reference variables of tilt, heading, pitch, etc. Radar file from shipborne radars, such as those in issue #194 exhibit this characteristic.

Currenrtly the RadarDisplay object will plot this data using the median latitude and longitude as an origin but for a correct visualization the movement of the platform must also be taken into account.