Eigen::Map fix in IMU_tools, using Eigen::Stride

This addresses core::#150, whose description is copied below:

Problem: call:

function(M.col(i));

with

function(MatrixBase<...> V) // In fact, V is supposed to be a column vector
{
  Map<Matrix<3,1> > p ( & M(0) ) ; // Fails because the memory is not continuous in M when taking one of its columns
} 

Solution: Avoid Map< ... > and create a copy of the variable.