* add access function to IndexIVF;

* - access for IndexIVF;
- write_index/read_index with std::function<...>;

* - fix test compile on mac;
- adjust write/read with std::function;

* replace std::function with IOReader/IOWriter;

* remove IndexIVF::access // tmp

* PFN_WRITE/READ => WRITE;

* revert mac compile fix;

* rename;

* fix compile;

* reset CMakeList;

* format; remove unused function/header;