peclet-flow 0.4.0

BREAKING: the pnm pore-network-extraction module moved to its own package, peclet-pnm (import peclet.pnm; was peclet.flow.pnm).
New: directional ghost-cell IBM projection (staggered + collocated, resolves 2nd-order collocated drag at curved boundaries; multi-rank), throat-safe collocated scheme (face_interp mode 9), get_ox_proj/get_oy_proj/get_oz_proj (the openness the projection conserves — for flux bookkeeping e.g. peclet.pnm), eps-conservative porous momentum/projection pair, distributed multiphysics validation, MG smoother/halo overlap, GraphAMG bottom solve.