feat: [linalg] add iterative solvers

[jalvesz]

This PR introduces the base for iterative solvers.

Two methods are proposed:

• Conjugate Gradient (CG)
• Preconditioned Conjugate Gradient (PCG)

Each method is made public with two public interface flavors:

• stdlib_solve_<method>_kernel: All arguments are mandatory (no optionals/no internal allocations), it contains the methods steps. The linear system (and preconditioner) is defined through a public DT stdlib_linop which enables to extend two key procedures: matvec equivalent to a matrix-vector product and inner_product equivalent to the dot_product. This is the interface recommended to extend the method when dealing with a matrix type not available in stdlib or when working in distributed-memory frameworks for which the matvec, dot_product and factorization steps need to be adapted to account for parallel synchronization.
• stdlib_solve_<method>: API with optional arguments, the linear system can be defined with dense or CSR_<>_type matrices. For the PCG, the following preconditioners are available: none (identity), jacobi (1/diagonal). It internally uses stdlib_solve_<method>_kernel.

• tests
• examples
• documentation

[jalvesz]

@jvdp1 thanks for your reviews. I think I managed to address all of them. Let me know your thoughts.

[jvdp1]

Thank you @jalvesz. LGTM.

[jalvesz]

With no more comments I'll go ahead and merge this.