Image color filter python demo fails

[albarji]

Upon running the demo_filter_image_color.py in prox-tv-3.1.1 the following error appears:

Traceback (most recent call last):
  File "demo_filter_image_color.py", line 22, in <module>
    F = ptv.tvgen(N,      [lam, lam],                  [1, 2],                   [1, 1])
  File "/home/alvaro/anaconda3/envs/python2/lib/python2.7/site-packages/prox_tv/_prox_tv.py", line 581, in tvgen
    ws = force_float_matrix(ws)
  File "/home/alvaro/anaconda3/envs/python2/lib/python2.7/site-packages/prox_tv/_prox_tv.py", line 217, in force_float_matrix
    if x.dtype != np.dtype('float64'):
AttributeError: 'list' object has no attribute 'dtype'