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.

[ptahmose]

This feature still seems valuable and desirable to me, but I'd choose to give it a a lower priority in the context of #73. Reasons:

• I'd expect only a modest performance increase for the specific use case
• I'd expect integration and orchestration in the context of pylibCZI as challenging and not-really-clear at this point.