0.2.8 release, new neural network, optimise, linalg, and etc.

full support of cblas/lapacke, redesign plot module, bug fix, perform…

…ance improvement

more support for neural network module

more functions in ndarray and matrix module, fix bugs

redesign operators, complete owl_ext module

improve performance and api, add more powerful slice function

new module structure

fix bugs, add more operations and math functions to ndarray, matrix m…

…odules.

add support for ndarray and sparse matrix

dense/sparse matrix of real/complex numbers