builder: easy way to save data from single trial

[jeremygray]

to save data from a single, unique trial, it seems like it should not require the user to make a loop with 1 repetition and no conditions. that seems like extra work and counter-intuitive. key response components allow you to specify what will be saved. then nothing is saved unless its part of a loop.

maybe there's a way to auto-detect whether a routine on the flow is not part of a loop, and implicitly make a trial handler with 1 rep (doing so invisibly, behind the scenes). probably a full blown trialHandler is not actually needed, and would produce more code than necessary in the lastrun.py files, but it might be a relatively low-effort way to invoke code that will save the data.