`first_plan()` feature to match `last_plan()`

[christopherkenny]

A symmetric feature here would be useful.

first_plan(plans)

[CoryMcCartan]

would it get the first plan in the object or the first sampled plan?

[christopherkenny]

I see why we haven't implemented it. I would think just column 1?

[CoryMcCartan]

Not quite sure the use case. Ref plans (& the init plan for MCMC) are right at the top and labeled nicely. last_plan() gets you the end of shortburst/ an MCMC chain / a random SMC plan (since those are shuffled)