DMStag: set_values! from reused buffers, length(is), on_lower_side

[filoferra]

Four small additions for DMStag assembly code (all additive)

• set_values! and zero_rows_local! pass view(buf, 1:n) to PETSc as the parent Vector and a count, so a reused scratch buffer allocates nothing
• length(is) is ISGetSize, the global size, as length of a PetscVec
• on_lower_side(loc, axis) says whether a location sits on the lower side of its element along an axis, without comparing against DMSTAG_* constants
• the with_field_views! docstring shows two DMs nested, with one vector handed back mid-scope for local_to_local!