clean_zeros -- Eliminate zero coefficients

    call a%clean_zeros(info)

Eliminates zero coefficients in the input matrix. Note that depending on the internal storage format, there may still be some amount of zero padding in the output.

Type:: Asynchronous.
On Entry
a: the sparse matrix.
A variable of type psb_Tspmat_type.
Scope: local.

On Return
a: The matrix a without zero coefficients.
A variable of type psb_Tspmat_type.
info: Return code.