Creating you own BoxMap type

Subtypes of the abstract type BoxMap must have four restrictions:

1. There must be a domain field within the type, i.e.

   struct MyBoxMap{N,T}
       domain::Box{N,T}
       # other things ...
   end

2. There must be a method map_boxes(g::MyBoxMap, source::BoxSet) which computes the setwise image of source under g and returns a BoxSet.
3. There must be a method construct_transfers(g::BoxMap, domain::BoxSet, codomain::BoxSet) which computes a dictionary-of-keys sparse matrix mat with mat[(hit_key, source_key)] = weight for the TransferOperator, where hit_key ∈ codomain.set and source_key ∈ domain.set.