Refactor david: add docs and standardize the code (#4734)

* Refactor david: move lagrange allocate operation from diag to constructor

* Rename pbasis -> basis

* update docs for hpsi and spsi

* update docs for David constructor and distructor

* Docs: change docs of DiagoDavid::diag

* Docs: cal_elem in david

* Comment out scc that is not actually used in DiagoDavid::diag_zhegvx

* Docs: cal_grad in david

* Docs: cal_grad in david

* diag_mock renamed diag_once in DiagoDavid

* Docs: linear algebra in cal_grad

* Docs: preconditioning in david

* Docs: refresh in david

* Docs: correct some comment

* Refactor refresh: Modify the 'dangling' (i.e. unused) parameter
 to ensure the function refers to the actual argument
 rather than directly accessing a global variable