Add vtkMap::ShowArea() to display rectangular area of interest
Update camera on vtkMap::ShowArea(), if renderer has been initialized.

Also:
* Changed code to keep Center coordinates in lat-lon
* Set zoom 1 level lower (undo last iteration)
Merge branch 'master' into john/show-area
Prune conus bounds a bit, so that default display has 1 marker  & 1 cluster
Rename vtkMap::ShowArea() to vtkMap::SetVisibleBounds()
Add GetVisibleBounds() method, to complement SetVisibleBounds()
Merge branch 'master' into john/show-area

Resolved conflicts:
	vtkMap.cxx
Merge pull request #64 from OpenGeoscience/john/show-area

John/show area