Enhance pmbasis

[gilvillard]

Slight modifications of nmod_poly_mat_middle_product_naive and nmod_poly_mat_pmbasis:

• Truncation at order order at the beginning of the basis computation, note that this improves execution times, as otherwise the initial order would be used for every recursive call. It seems that this also allows to use nmod_poly_mat_middle_product_geometric successfully.
• A naive middle product that seems better suited to the very common situations with unbalanced degrees.

The following comparisons using both nmod_poly_mat_pmbasis and nmod_poly_mat_kernel should be convincing.

compare.pdf