scanning/scan.py: fix get_plot()

(added WIP: because tests are missing, but the bug is fixed)

As @sebastien.petitdemange already found in issue #434 (closed), get_plot() was broken since groups are named group_N instead of the concatenation of axes names in the group. However, the reason of the bug was more deep: in fact, it worked before by chance, indeed the code was completely forgetting about master channels (sorry, my bad !). For example if get_plot() were called before with roby but group name was roby/robz it would not have worked. It would have worked for robz/roby because first scalar channel would have been like robz/roby:channel_name.

I will add the necessary tests to make sure get_plot() behaves like we want with different combinations.

Fix issue #434 (closed)