In test_feature, no reversion of coordinates

Internally, coordinates are now processed in y, x or z, y, x order. This
to have the same dimensional order as in image.shape and to stop me
getting more and more confused on whether to reverse coordinates or not.
Anisotropic feature tests + some enhancements

- made feature tests compatible with anisotropic features in 2 or 3
dimensions
- draw_gaussian_spot only calculates the gaussian in a square with size
6*radius, this greatly increases speed.
- added a margin option in gen_random_locations, so that generated
features will not be missed by trackpy.locate
- added test_one_centered_gaussian_3D_anisotropic
- added test_multiple_anisotropic_3D_simple