Expose box constraints as intervals

As the changes to obstacletnnmgstep.hh illustrate, this allows us to replace code like

```c++
{
    if (coarseCorrection_[i][j] < defectConstraint.lower(j))
        coarseCorrection_[i][j] = defectConstraint.lower(j);
    if (coarseCorrection_[i][j] > defectConstraint.upper(j))
        coarseCorrection_[i][j] = defectConstraint.upper(j);
}
```
with
```c++
    coarseCorrection_[i][j]
        = defectConstraint[j].projectIn(coarseCorrection_[i][j]);
```
which is no less efficient yes far easier to read.

I have no removed any functionality. The set of features that a `BoxConstraint` currently comes with is quite odd, though. It ha an `operator-=` but no `operator+=` e.g.

See merge request !6