Add a way to change the BASEURL

[nabobalis]

There are several (I am unclear how many) mirrors for Helioviewer and people might want to use a mirror instead of the main URL. So we will need to add a way to change this either as a keyword, a configuration option or even a system environment variable.

[wafels]

There are at least three URLs.

[nabobalis]

Do you want us to test against all three in our tests?

[wafels]

Not a bad idea from the point of view of testing the different locations. Two of them GSFC controls, and the other is in France. Our French colleagues may also have some interest in this project too.

[nabobalis]

We also have the beta URL: api.beta.helioviewer.org