update nodal_stress() to accept user-provided integrals
new examples/linear_elasticity/its2D_interactive.py
docs: add interactive example section to primer
Merge branch 'its-interactive-example'
docs: update user's guide for interactive probing example in primer
script/gen_gallery.py: omit examples that use VTK probes

- bad interaction, probably with Mayavi, leads to segfault
docs: fix IGA section for current sources