John/show area

Adds vtkMap::ShowArea() method to set the display to contain a specified geo-rectangle.

Perhaps the code is not the most intuitive, but the objective is to find the smallest zoom level (highest number) that is still bigger than the size of the input coordinates. I could have taken the zoom == 0 size (360.0) and divided down, but instead took the size of the area (delta) and multiplied up.