Introduction of _krylov_tools.py for Unified Krylov Framework (#434)

* a general Krylov func, can solve eigenvalue & lienar equation & shifted lienar equation; a nested-krylov framework, allowing non-diagonal (iteratively solved) preconditioner for faster Krylov convergence

* explicitly assign the initguess_fn & precond_fn; a separated test_krylov file

* improve docstring, more readable; add test for single precision

* add example usage of krylov_solver

* convergence threshold for single precison eigenvector cannot be too tight

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Co-authored-by: zhouzehao.dbsv <zhouzehao.dbsv@bytedance.com>

Author: John-zzh