* Change DataReader::read()'s signature to fix warning C4267

This CL fixes lots of warning "C4267: 'initializing': conversion from
'size_t' to 'int'" in our codebase by matching DataReader::read()'s
signature to fread().

* Fix warnings C4244 and C4267 in tools/ncnnoptimize.cpp

C4244: 'initializing': conversion from 'double' to 'float',
    possible loss of data
C4267: 'initializing': conversion from 'size_t' to 'int',
    possible loss of data

* Fix warning C4244 in src\layer\selu.cpp

C4244: '=': conversion from 'double' to 'float', possible loss of data

* Fix warning C4244 in src\layer\cast.cpp

C4244: '=': conversion from 'double' to 'float', possible loss of data
C4244: 'return': conversion from 'float' to 'signed char', possible loss of data

* Fix warning C4244 in src\layer\psroipooling.cpp

C4244: 'initializing': conversion from 'double' to 'float',
    possible loss of data
C4244: 'initializing': conversion from 'double' to 'int',
    possible loss of data

* fix onnx2ncnn MemoryData dims problem #1330

* fix ncnn2int8 bug (see #1336)

* fix bug that MemoryData as multi operations input

* fix ncnn2table find_blob_index_by_name data failed bug, see #1338

* fix ncnn2table find_blob_index_by_name data failed bug, see #1338, fix change requested

* fix Reduction operation attribution bug

* fix #1363 request changes

* [WIP] add reduce op support for onnx

* extend reduction to support 1,2-dim reduction and keepdims

* fix compile error

* split type to 3 flags && split keepdims to another function

* add relative README_CN.md;
* obtain time cost with op->forward().