Auto-box single values for is_many properties in creation.

This creates more symmetry between get() and create() when passing
single values for an is_many property.

Two motivating cases:
*AnP wherein people often specify an "incorrect" config by forgetting when [] are required versus not required when only specifying one value for their model creation.
*ClinSeq wherein the Workflow currently has duplicate properties, e.g. "build" and "build_as_array" depending on the whims of the commands used in the workflow. ("Workflow classic" did this autoboxing, but Ptero does not.)

[Apologies for accidentally pushing first instead of opening this request. I was trying out the new Pull Request docs and accidentally set it up exactly backwards.]

There may be consequences of this change that I'm not thinking of.  It seems like this pull request could be a convenient place to have a discussion about the idea?