Explain usage of map_types_to_class()

[ablaette]

I wonder whether the function should really be exported. As of now, its usage is unclear, without a telling example.

[ChristophLeonhardt]

I added some more documentation to the function (on devel). An explicit example would probably still be a good addition.

[ablaette]

On a temporary branch 'entity_types', I drafted an S4 method entity_types_map(). The idea is to follow a noun-verb naming convention. We would then have a list-method and a data.table-method.

The transfer to the method is not consolidated - this initial step is about the look & feel.