Add expand.only option to return the expanded data without running models

[remlapmot]

As per Alejandro's email I think we need this option.

SEQopts(expand.only = TRUE) makes SEQuential() return the expanded data.table immediately after expansion (and factorize), skipping weights, outcome modelling, KM/risk and bootstrap.

Obvs FALSE by default.

[ryan-odea]

This looks good! There is also the option of exposing SEQexpand. Although this is perhaps cleaner

[remlapmot]

Thanks Ryan - interesting point - yes I think exposing SEQexpand() would mean exposing some other stuff, so I'll leave this as is for now.