[MRG] Remove packaged GeNN

[mstimberg]

As discussed in #53, this removes our fancy "pre-compile GeNN libraries and ship them with BrianGeNN" approach. Users should install GeNN manually and then install Brian2GeNN from source via pip.

Closes #53

[mstimberg]

@tnowotny , @thesamovar this is fine with you, right? After merging, I think it would be time to do the 1.2 release (given that we only release the source package, doing the release will be straight-forward...).

I think I'll also move our existing conda packages to an archive label or something like that. This will prevent users from installing an outdated version by following older installation instructions.

[mstimberg]

PS: Don't worry about the build failure, we test everything twice (once on the branch and once after merging -- given that nothing else changed on the master branch, the two are perfectly identical) and one of the two ran fine, the other took to long for the appveyor timeout.

[tnowotny]

Yes, as discussed, I think this is the most sensible option. I would hope it leads to a lot less trouble with compiler and library versions etc
so, ok to go forward from my perspective.

[thesamovar]

Yeah, it's a shame to not have it be one line install, but that's kind of unrealistic anyway and we don't want to have a heavy maintenance burden on this.