More DMStag: local_to_local!, with_product_coordinates, set_matrix_preallocate_only!

[filoferra]

Stacked on #295 : review that one first. Until it merges, this diff includes its commit.

The rest of what DMStag usersstill might do through LibPETSc:

• local_to_local!(dst, dm, src) and the in-place local_to_local!(v, dm) refresh the ghost points of a local vector from its neighbours. Tested on 4 ranks.
• with_product_coordinates(f, dm) gives a block read-only access to a DMStag's per-axis coordinates (x[i, 1] is the lower face of element i, x[i, 2] its centre) and hands them back when the block returns or throws.
• set_matrix_preallocate_only!(dm, true) makes a matrix from dm get its nonzero pattern from the first assembly, instead of storing every coupling the stencil allows as a zero. It's a setter rather than a PetscMat keyword because the flag stays on the DM.

Fixes:

• LibPETSc.DMStagRestoreProductCoordinateArraysRead called the writable restore, which doesn't match the read-only get.
• The KSP and SNES docstrings now say when the constructor's options are applied: once at construction for KSP, at every solve! for SNES (and TS, whose docstring already said so).

[filoferra]

Added a third piece to this PR: with_field_views!(f, dm, vecs...; fields, read, write).

It checks out DMStag local vectors and gives the block one view per field (location => dof), indexed by element like stencil, ghosts included:

flow = (face_location(dm, 1) => 0, face_location(dm, 2) => 0, element_location(dm) => 0)
with_field_views!(dm, x, r; fields = flow, write = (false, true)) do (Vx, Vy, P), (Rx, Ry, Rp)
    ...
end

Without fields, the block gets the whole [I..., slot] array. read/write work as in with_local_array!.

• The views are concretely typed, so user code no longer has to spell out PETSc.jl's internal array types.
• A checkout costs 9 allocations per vector, the same at 8² and 64².
• An exception in the block hands every vector back. A global vector, or one from another DM, throws ArgumentError.

Tests: test/dmstag_views.jl.