Accelerate x3 linear transform interpolation

Use Eigen::Isometry3d::linear() method instead of Eigen::Isometry3d::rotation() to avoid performing useless SVD.

This brings a nice x3 speedup, from ~1.5 µs to 0.5 µs on my machine.

Fixes #13 (closed).

@julia.sanchez @samkys Please take a look.