Tikhonov regularization with Identity Tikhonov matrix

Add a simple extension of the gradient descent method for solving Tikhonov regularization of the form:

Example of usage:

recon = tomopy.recon(
    proj, theta, center=rot_center, algorithm='tikh',
    reg_data=v, reg_par=[-1, alpha], num_iter=32,
) # where -1 refers to automatic gradient step search