Extend preallocation

[ntardieu at giref.ulaval.ca]

Dear Petsc users,

I would like to compute the sparse projection matrix Q=Id - Pt*A*P.

Pt*A*P is very well computed by MatMatMult ; then I would like to use
MatShift in order to complete the computation.
Unfortunately, Pt*A*P has lots of zeros on its diagonal, thus MatShift is
very slow due to inapropriate preallocation.

Since the initial preallocation of Pt*A*P is very good, I would to know if
there is a method allowing to get the allocation data structure and to
extend it in order to preallocate the diagonal terms.

Best regards,

Nicolas