v1.5.0

Physical gradients only + ParaView export only via writers (breaking). derived_fields() always estimates true gradients with the real (dx, dy, dz) spacings; the negated-u / dx := dz convention left postptv for openptv-analysis (private). Legacy raw-vtk writers (eulerian.export_vtk, export_vtk_rectilinear_series, vtk_export.py) removed; all ParaView export goes through flowtracks.writers (.vti/.vtr/.vtp + .pvd).