net_download should support XNAT

[tvercaut]

I am transferring cmiclab issue 198 to GitHub as @atbenmurray expressed interest in helping improve support for XNAT in NiftyNet.

As initially discussed with @tomdoel @tvercaut and @dzhoshkun, it would be good for net_download to support fetching data from an XNAT server. This would allow us to easily share data and models internally (and externally, if the group sets up a public XNAT server) by uploading to an XNAT project.

We have python code for downloading data from XNAT. The main issue according to @tomdoel was deciding how to deal with authentication.

(A similar mechanism could be used to fetch code from a GitHub or GitLab server.)

[tvercaut]

As discussed with @tomvars and @mjorgecardoso a similar requirement for TCIA and Synapse would help making better use of challenge-related datasets.

[atbenmurray]

@wyli and I had a discussion related to memory-based images #346 and he is modifying the implementation of #346 accordingly. This should provide the framework for how to handle this and other input-source-related items