Align/Rotate Point Cloud Along Z direction based on PCA

This function rotates the point cloud of the highest eigenvector
to be along z-direction.
For the 2nd highest eigenvector, the user can choose to align either
along x or y direction

parameters:
inputs:
pc: point cloud of size N * 3 size
direction: 'x' or 'y' (align the 2nd highest eigenvector along 'x' or
'y' direction

output:
pc2: rotated point cloud