BUGFIX: make sure chain tracks all points

It looks like two bugs crept into !1187 (merged) just before it was merged.

• one is the ordering of the updates of the bounding ellipses and the MCMC length for the LivePointMultiEllipsoidSampler. The fix here means that the scale attribute will update as expected within dynesty.
• the ACT tracking broke because points weren't actually being added to the chain during the loop.

I also removed an excess full ACT calculation as nothing is bigger than inf!