Allow syntax observer(; f, g)

[mtfishman]

Allow the constructor syntax observer(; f, g), or observer(; f=func_f, g=func_g).

The name of the column will be the name of the keyword argument. I think I didn't do allow this before because currently keyword arguments passed to observer get forwarded to the DataFrame constructor call, however we can disambiguate that with the syntax observer(; f, g, dataframe_kwargs=(; makeunique=false, copycols=true)). This will preclude using dataframe_kwargs as a column name if that syntax is used, which seems like a reasonable compromise. Supported keyword arguments for the DataFrame constructor can be found here: https://dataframes.juliadata.org/stable/lib/types/#DataFrames.DataFrame.

[mtfishman]

Closed by #21.