cleanup facet jump confusion

Remove the sign confusion regarding jumps and facet normals. Looking at the literature revealed that both versions are used. Sometimes '-' refers to self and sometimes '+' refers to selft. In the end it doesn't really matter as long as it is consistent with the definition of the jump and if you make the correct choice (outer unit normal) on the boundary.

In UFL the situation is as follows:

• jump(v) = v('+') - v('-')
• The corresponding outer unit normal pointing from ('+') to ('-') is FacetNormal(cell)('+').

This means we should use ('+') as self and ('-') as neighbor in dune-perftool. We did that for the outer normal vector but not for function evaluations. This merge request fixes the latter one and adjusts the DG tests.

This resolves issues #103 (closed) and #105 (closed).