Revive having `timeseries` for all readers and /or having a `transfer_to_memory` function at the reader level.

[hmacdope]

Is your feature request related to a problem?

#2271 and #1400 first raised the idea of having a timeseries attribute for all readers. Currently timeseries is available only for DCDReader and MemoryReader causing all kinds of problems particularly with ENCORE. See #3761 MDAnalysis/mdaencore#30 #2948 #2578 and others.

Describe the solution you'd like

IMO the simplest way to do this is with a method attached to ProtoReader that returns the full in-memory representation of the trajectory. This can then be overridden if there is a faster way, eg with a Cythonised reader etc.

This can also then tie in with the memory-reader nicely if possible. I am no memory-reader expert so would love some guidance on how this might tie in @richardjgowers.