Improve @transform macro to handle obj.field expressions

[juliohm]

Currently, the following code doesn't work:

@transform(geotable, :column = coords(:geometry).x < 100km)

The workaround consists of defining the function outside the macro:

fun(point) = coords(point).x < 100km

@transform(geotable, :column = fun(:geometry))

Basically, the expression obj.field is converted to obj.:field and incorrectly interpreted as a column of the geotable.

We need to handle this special case like other packages do (e.g., DataFramesMeta.jl)