Allow reading directly from HTTPS endpoint using fsspec

[hmacdope]

We should add an example of reading from a raw URL, no AWS.

[orbeckst]

You could use files from the AdK equilibrium dataset from FigShare https://figshare.com/articles/dataset/Molecular_dynamics_trajectory_for_benchmarking_MDAnalysis/5108170/1 — the PSF and DCD file should be directly accessible.