Add transducer and GPS offset correction

[CameronBodine]

If an external GPS is not available, or if it is not placed directly above the transducer, then the coordinates for each ping are off by the distance to the control head. The coordinates can be easily corrected based on the x and y offset between the GPS and the transducer. The offsets can be measured based on the boat's coordinate system:

• The origin (0,0) is the location of the control head, or external GPS (if being used)
• The x-axis is from the bow (fore, or forward) to the stern (aft, or rear), with positive offset values towards the bow, and negative values towards the stern.
• The y-axis is from the portside (left) to starboard (right), with negative values towards the portside, and positive values to the starboard.

Once the smooth trackline is generated, the course over ground (COG), x, and y-offset will be used to calculated the northing and easting positional offsets.