Refactoring SampledBoxMap

[April-Hannah-Lena]

This is the update to using FLoops.jl. There are some breaking changes:

• domain_points and image_points for SampledBoxMaps now return points within the box. The caller must ensure that this holds, eg by using the new convenience functions center(b::Box) and vertices(b::Box), also rescale(center, radius, points) which returns a generator that rescales points within [-1,1]^N to be within Box(center, radius). Constructors such as BoxMap, PointDiscretizedMap, AdaptiveBoxMap use these functions.
• map_boxes now uses @floop. Every boxset.set must implement a SplittablesBase.halve method. The base julia Set has this, and methods for OrderedSet, BitSet will be added in the next PR through SplitOrderedCollections.jl
• There is a temporary working TransferOperator for gpu boxmaps, but it is not very elegant or efficient. The "real" gpu TransferOperator method will be added in the next PR, which is the TransferOperator refactor.

[gaioguy]

Interesting construction. I wonder whether this might be useful for the TreePartition as well.