Add expand_only option to SEQopts to return expanded dataset without running full analysis

[remlapmot]

Adding the same option as per today's SEQTaRget PR.

[ryan-odea]

I am a bit mixed on including this in the python version - I suppose it is not bad to include additional options to the user. It's not too difficult to inspect the class object after expansion has been applied (collect should be able to handle this at any time).

Wondering about your thoughts?

[remlapmot]

I was just trying to keep the R and Python packages with approx the same options.

One difference/benefit this gives beyond skipping the computation: in expand_only mode the protocol-censoring step (_dynamic) is also skipped, so the returned DT is the raw expansion — which you can't get by just reading s.DT after a normal expand().

We already have plenty of options, so one more doesn't add much overhead.

So I'm still in favour of this.