preconditioner for matrix-free linear system

[Satish Balay]

On Tue, 14 Apr 2009, Matthew Knepley wrote:

> I think all the PCs you want rely on AIJ storage. People in linear
> algebra don't think much of generic interfaces, and thus do not
> write their preconditioners to respect them (or you would only have
> to implement GetRow()).

If a user provides a MATSHELL with MatGetRow() interface - perhaps we
should be able to provide a generic MatConvert() interface as well [to
MATSHELL] - that gives a AIJ matrix - useable for PMat? [with access
to all PCs]

[To get efficient assembly we might have to call MatGetRow() twice?]

Satish