Feature: add row-major in DensityMatrix::_DMK

[linpeize]

now DensityMatrix::_DMK supports both row-major and col-major

[mohanchen]

Can you explain when do we need row-major DMK?

[mohanchen]

As a PR reviewer, we often passively receive new feature submissions from developers without clear explanations of their necessity. This PR introduces an extra logic layer to distinguish row-major and column-major orders, which greatly complicates the existing code. For such pull requests, I always question whether it is reasonable to add this complexity to the original implementation.

[linpeize]

As a PR reviewer, we often passively receive new feature submissions from developers without clear explanations of their necessity. This PR introduces an extra logic layer to distinguish row-major and column-major orders, which greatly complicates the existing code. For such pull requests, I always question whether it is reasonable to add this complexity to the original implementation.

At present, DMK uses Fortran-style column-major order. This poses unnecessary difficulties for developers.
I have added an option, hoping that future development will adopt C++ row-major order wherever possible.
For now, all actual calls still follow the column-major order.