Add a new Layer "DataCell" to manage data access

[LHT129]

I think high level index should ignore the specific implement of quantization and io type, So the quantization and io components can be managed by new layer DataCell which provide different data access interface to the index layer. just like the following show

Layer	Description	Implement
Index Layer	ANN algorithm & architecture.	Graph (HNSW,NNDescent) + Reorder, IVF(PQ,SQ,fastscan), HNSW_IVF, MultiIndex ......
DataCell Layer	data access, use quantization, io and others	FlattenDC, GraphNeighborhoodDC, MixtureDC .......
Operator Layer	quantizations, IO and other basic components	PQ,SQ,MemoryIO,SSDIO,......

this abstract layered approach may facilitate scalability and code reuse currently we only have index layer, so the issue will provide Datacell Layer and Operator Layer in this feature. the related pull requests:

• 25

• 34

• 37

• 41

• 35

• 59

• 60

• 61

• 62

• 79

• 80

• 96

• 111

• 114

• 125

  todo tasks:

• support mixture DataCell
• adapt Graph DataCell for HNSW
• implement HNSW + SQ4 + SQ8 Index

[LHT129]

Maybe like this