second try at implementing support for crystallographic density maps

modules/base/pymod/export_symmetry.cc

modules/base/pymod/export_symop.cc

modules/base/pymod/export_unit_cell.cc

modules/base/src/symmetry.cc

modules/base/src/symmetry.hh

modules/base/src/symop.hh

modules/base/src/unit_cell.cc

modules/base/src/unit_cell.hh

modules/base/tests/test_symop.cc

modules/base/tests/test_unit_cell.cc

modules/gfx/pymod/__init__.py

modules/gfx/pymod/export_map.cc

modules/gfx/src/impl/map_octree.cc

modules/gfx/src/impl/map_octree.hh

modules/gfx/src/map_iso.cc

modules/gfx/src/map_iso.hh

modules/img/alg/src/correlate.cc

modules/img/alg/src/density_slice.cc

modules/img/alg/src/gaussian_gradient_magnitude.cc

modules/img/alg/src/gaussian_laplacian.cc