PCShell interface

[Jed Brown]

With MatShell and the SNES/TS interfaces, the user's callback gets the
PETSc object, but this is not so in PCShell.  I find this pretty clumsy,
for example, since there is no PC, the user preconditioner can't call
PCGetOperators.  This means that in contrast to MatShell, the user
really can't write first-class PC implementations using PCShell.  It
also appears that it is not possible to implement PCApplyBAorAB without
obtaining the operator through some other channel.

Perhaps it's better in the long run to always write a full-blown
implementation (i.e. copy jacobi.c and modify), but I think it would
still be more consistent if PCShell passed the PC to the user's
callbacks.  What do you think?

Jed

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