When an asset is represented as directory in Blender (for example fluid
simulation caches), that directory is traced and each file is considered
an asset.
This makes it considerably easier for Shaman clients, as they need to
compute the SHA256 checksum of each file. The logic to transform a
directory path to a list of the contained files is now in BAT itself.
This does introduce some not-so-nice things, like having to annotate each
`__init__` function with `-> None`. However, the benefits of having static
type checking in a complex bit of software like BAT outweigh the downsides.
