improve quality control for coregistration, segmentation and meshing of EEG head model

as discussed with @canoni and @mcpiastra
 
The figure 2 in the EEG BEM headmodel tutorial should be redone and include the plotting after reading, segmenting and meshing. Looking at the mesh after the headmodel is constructed is too late.

Furthermore, the tutorial should include the actual figures and it should point to FAQs that explain what to do if something is not as it should be.

We should also consider including the estimated segmentation volumes in the QA.