Add clipping to MultiplyRobust estimator

[brash6]

In the med_bench_prototype implementation, clipping is not implemented, there's only these commented lines :

# clipping
# p_x_clip = p_x != np.clip(p_x, self._clip, 1 - self._clip)
# f_m0x_clip = f_m0x != np.clip(f_m0x, self._clip, 1 - self._clip)
# f_m1x_clip = f_m1x != np.clip(f_m1x, self._clip, 1 - self._clip)
# clipped = p_x_clip + f_m0x_clip + f_m1x_clip

# var_name = ["t", "y", "p_x", "f_m0x", "f_m1x", "mu_1mx", "mu_0mx"]
# var_name += ["E_mu_t1_t1", "E_mu_t0_t0", "E_mu_t1_t0", "E_mu_t0_t1"]
# n_discarded = 0
# for var in var_name:
#     exec(f"{var} = {var}[~clipped]")
# n_discarded += np.sum(clipped)

Do we want to perform clipping ?
If yes, do we want to uncomment these lines or do we want to perform it in another way ?
@houssamzenati @bthirion @judithabk6

[bthirion]

+1 for it...
... but preferably in a forthcoming PR.