Method hierarchy

This method builds the hierarchy of matrices and restriction/prolongation operators for the multilevel preconditioner p, according to the requirements made by the user through the methods init and set.

`a`	`type(psb_`x`spmat_type), intent(in)`.
	The sparse matrix structure containing the local part of the matrix to be preconditioned. Note that x must be chosen according to the `real`/`complex`, single/double precision version of AMG4PSBLAS under use. See the PSBLAS User’s Guide for details [21].
`desc_a`	`type(psb_desc_type), intent(in)`.
	The communication descriptor of `a`. See the PSBLAS User’s Guide for details [21].
`info`	`integer, intent(out)`.
	Error code. If no error, 0 is returned. See Section 7 for details.

5.3 Method hierarchy_build