prepare for v2.9.1

prepare for GPUMD-v2.9

Merge pull request brucefan1983#111 from brucefan1983/normalize

Get the descriptor normalization back

remove unused code

remove unused code and update a readme

update readme

remove png figures in examples (new figures are shown in Jupyter note…

…books)

correct file path (before this, not working in Linux)

correct some bugs in SBOP

I am still not very clear why the pointers b, bp, f12x, f12y, and f12z cannot be members of the SBOP class. Because the SBOP class is created in the heap instead of the stack?

Update README.md