add new metrics to API

skimage/measure/__init__.py

@@ -2,6 +2,7 @@ from ._find_contours import find_contours
 from ._marching_cubes import (marching_cubes, mesh_surface_area,
                               correct_mesh_orientation)
 from ._regionprops import regionprops, perimeter
+from ._simple_metrics import mse, nrmse, psnr
 from ._structural_similarity import structural_similarity
 from ._polygon import approximate_polygon, subdivide_polygon
 from ._pnpoly import points_in_poly, grid_points_in_poly
@@ -34,4 +35,7 @@ __all__ = ['find_contours',
            'profile_line',
            'label',
            'points_in_poly',
-           'grid_points_in_poly']
+           'grid_points_in_poly',
+           'mse',
+           'nrmse',
+           'psnr']