Merged LineModel3D with LineModel in order to form a unified model.

- model.params holds the legacy params (2D representation),
 - model.new_params holds the true ND line representation.

The proposed LineModel behaves identically as the implementation in master in the 2D case. In the 3D case, it behaves similarly to the previously proposed LineModel3D, as long as new_params is used instead of params. This implementation thus takes advantage that the 2D case is a special case of the ND general model.

doc/examples/plot_ransac3D.py

@@ -10,7 +10,7 @@ the RANSAC algorithm.
 import numpy as np
 from matplotlib import pyplot as plt
 from mpl_toolkits.mplot3d import Axes3D
-from skimage.measure import LineModel3D, ransac
+from skimage.measure import LineModel, ransac
 
 np.random.seed(seed=1)
 
@@ -26,7 +26,7 @@ xyz[::2] += 20 * noise[::2]
 xyz[::4] += 100 * noise[::4]
 
 # robustly fit line only using inlier data with RANSAC algorithm
-model_robust, inliers = ransac(xyz, LineModel3D, min_samples=2,
+model_robust, inliers = ransac(xyz, LineModel, min_samples=2,
                                residual_threshold=1, max_trials=1000)
 outliers = inliers == False