[metric] design when computing a metric

[sim51]

When we compute a metric, a notification is displayed to say that the computation has been done, and we stay on the form where the message A node attribute named "modularityClass" already exists. Its values will be overridden. under the field attribut name. and that's all.

IMHO we have two choices :

1. close the form and reset the metric selected in the selectbox
2. display a message on the form
3. other ?

[paulgirard]

This issue is linked/duplicated with #31