Refactor MeshInterpolator

So that we can use it to interpolate to arbitrary nodal points sets. Before we could only use a second mesh type for the interpolation.

This refactoring also revealed that the way in which the extends of a tree's boxes were constructed was not accurate enough. In particular, find_cell_indices could fail when one asked for the end point of a mesh with extends [-1,1], because that was wrongly stored as 0.999999.