allow for "concurrent decoding" for tile-composition

[ptahmose]

c.f. #73 - when doing a tile-composition within accessors, it is desirable to perform a decompression-operation concurrently. This should speed up the operation considerably (assuming the availability of multiple cores i.e.).

The implementation should be not compromise the versatility of libCZI. Best case, the libCZI-codebase should not be tied to a concurrency platform, and imperative threading should also be avoided.