properly close VGP frame so it can be reopened
properly close VGP frame so it can be reopened

The close event of the VGP Viewer frame was not fully bound to the
parent frame's `vgp_open` attribute such that the viewer could not be
reopened within the same session after it was open and closed once.

lfairchild/PmagPy,lfairchild/PmagPy,lfairchild/PmagPy