data_frame/assemble: support factor + reference level

[mschubert]

In data_frame/assemble:

Creating data.frames that are used in models (lm, etc.) often require factors instead of character vectors.

There should be a convenient function to specify

• which characters to convert into factors
• which level of the factor should be the reference level

An easy way would be to add an option factor_refs = list(varname = "reflevel")