Provide two modes for the cut through mayavi picker option

[ykempf]

In some cases one wants a continued line when clicking more than twice, as is done now, but in other cases one would like to be able to get a completely detached new line.

I see two options:

• either a way of resetting the picks to start from scratch again
• or a second picker for detached lines [or something like a check box to choose from two behaviours].

Currently the workaround is to empty the parameter field, press enter, re-enter the command plot rho B,magnitude for instance and click again, after having also deleted the previous cuts.