added Reset to gfx.Entity, plus some other minor interface tweaks

modules/gfx/pymod/__init__.py

modules/gfx/pymod/export_entity.cc

modules/gfx/src/entity.cc

modules/gfx/src/entity.hh

modules/gfx/tests/test_gfx.py

modules/mol/base/pymod/export_query_view_wrapper.cc

modules/mol/base/src/query_view_wrapper.hh