Docstrings and TypeHints corrections

[syedalimohsinbukhari]

The following modules need doc-strings and type-hints corrections,

__backend

• [x] iterative_solvers_
• [x] predictor_corrector_

  iterative_solvers.system_of_equations

• [x] gauss_jacobi
• [x] guass_seidel
• [x] predictor_corrector_methods

  iterative_solvers

• [x] ~polynomial~ -> polynomial_roots
• [x] ~non_polynomial~ -> function_roots

[syedalimohsinbukhari]

Added doc-strings to polynomial_roots and function_roots