The mapper maps correctly, the issue was in the test where some of the
global points were not correctly identified due to numerical
inaccuracies, so there were duplicate entries like
[-1.0416666666666667407 -1.4583333333333332593], {45}
[-1.0416666666666667407 -1.4583333333333330373], {45}
in the global langrange point map.

This is obsolete now, as we can just access the local finite element to
obtain the lagrange points.

* document template arguments by being more verbose
* drop DofFieldType from interpolation
* export dimensions and types

GCC, for some reason, is perfectly fine with this missing member,
although it's explicitly accessed in a test!