VFE/DTC should be moved to ApproximateGPs.jl

[theogf]

No description provided.

[willtebbutt]

I keep changing my mind on this. On the one hand, the name suggests that this is a good idea. On the other, VFE / DTC / FITC don't add any additional deps to AbstractGPs, and the approximate posteriors produced satisfy the AbstractGPs API.

I don't think I would get in the way of this move, but I can also live with the status quo.

[theogf]

Well in my naive user experience, I kept looking in ApproximateGPs for it before realizing it was in AbstractGPs after asking on Slack.

[willtebbutt]

Haha fair enough. If you didn't know it was there, we should probably just move it here.

[ngiann]

Just to add my 2 cents (far more naive than @theogf): After quite some digging to figure out where does Stheno find VFE, I was quite surprised to find it in AbstractGPs.