Generalize inbound array converter

[wpbonelli]

The array converter that runs on component instance init should work not only on dictionaries but any data structure that we want to support converting to an array (e.g. nested list)

[wpbonelli]

cf #228, the converter should raise an informative error for invalid forms of the various supported syntaxes

[wpbonelli]

this is a required step for full support for #201