Enable specification of centering information via URL

[Jack Ireland]

Currently, the link function in HV describes the content of each layer, but does not describe what is in the viewer window. A use case is that the user is zoomed in and is looking at something far from disk centre, but also has both the coronagraphs included, e.g.,

http://www.helioviewer.org/?date=2010-04-28T23:00:00.000Z&imageScale=5.26&imageLayers=[SOHO,EIT,EIT,195,0,100],[SOHO,LASCO,C2,white-light,1,100],[SOHO,MDI,MDI,continuum,1,50],[SOHO,LASCO,C3,white-light,1,50]

Pasting this link in to another browser does not exactly reproduce the original FOV (field of view). Under the assumption that the feature of interest is close to the centre of the FOV, then including this information improves the chances that users can swap links that actually point out what they want. The centre of the FOV can also be used as the centering information in JHV, and in JHV solar rotation tracking.

[Keith Hughitt (keith-hughitt)]

I'm in the middle of refactoring the Viewport code to make it more flexible and simpler to work with. When I'm done, this will be much simpler to implement.