Allow `dict_list` to return other types than dictionaries

In many cases users may want to have the configuration parameters of their programs to be a dedicated Julia composite type. This has many advantages, including customizing savename. It would be nice if dict_list could create objects of a given composite type instead of dictionaries. Would also make the whole integration with produce_or_load simpler.

A very simple, but probably not the most performant way to do this, is to leave dict_list as is, and add a final conversion step by using our functions dict2struct and/or ntuple2struct.