Fix meshplot `offline` setting in segmentation viewer

[dmsteck]

Running segmentation_viewer.py as instructed in the readme file,

python -m tools.segmentation_viewer.segmentation_viewer \
   --meshes_folder s1.0.0/meshes                        \                      
   --output_folder ./tmp

causes two html plots to be generated for each example:

Plot saved to file 0534717d-d1e4-4bd4-bca4-5668781708a0.html.
Plot saved to file tmp/83226_3e4d1716_0.html.
Plot saved to file bdaadfe1-6811-4a61-8f96-1f1b558c9871.html.
Plot saved to file tmp/21901_64acc24c_8.html.
Plot saved to file 06b0dfd4-bda3-44fc-910b-63cb2c534e76.html.
Plot saved to file tmp/21900_760d2078_21.html.
...

This is because meshplot.plot creates an additional html file when in "offline" mode, see here.

This PR fixes that by calling mp.website() instead of mp.offline().