[WIP] Expanding the BEM surface when they intersect.

[jaeilepp]

Partial fix to #3291

I was playing around with the plotter in #3778 and wanted to try this. This is how it looks for sub002 of the biomag demo.

Before, calling make_bem_model would fail because the outer_skin is inside the outer_skull. This moves the outer_skin slightly outwards.

I'll let you decide if we want something like this.

[jaeilepp]

It's probably hard to figure out from the picture since the face is punched in for anonymity. I think the expansion happened on the left side (lower left part that's sticking out a bit).

[codecov-io]

Current coverage is 86.36% (diff: 41.17%)

Merging #3785 into master will decrease coverage by 0.01%

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  Lines         61111      61121    +10   
  Methods           0          0          
  Messages          0          0          
  Branches       9337       9342     +5   
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  Hits          52786      52786          
- Misses         5618       5627     +9   
- Partials       2707       2708     +1   

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[agramfort]

I am not convinced it's the right fix. I guess many scenarios can happen depending which surface intersects with what.