fix: use fixed-size Eigen vectors for plane-fit members

normal_, singular_values_, and pc_mean_ were declared as Eigen::VectorXf
(dynamic-size). When a partial (non-360°) point cloud is processed
— e.g. an OS1 128 Ouster pointcloud that doesn't start at azimuth 0 —
the dynamic vectors are accessed at fixed indices before being sized,
producing an immediate crash at startup.

Switch all three to Eigen::Vector3f. Matches the SVD output (Vector3f
from JacobiSVD<MatrixX3f>) and the existing per-index accesses
(e.g. singular_values_.minCoeff(), pc_mean_(0..2), normal_(0..2)).

Co-authored-by: MatteGombia <matteogombia@gmail.com>

Closes #62

Author: MatteGombia

cpp/patchworkpp/include/patchwork/patchworkpp.h

@@ -188,10 +188,10 @@ class PatchWorkpp {
 
   double d_;
 
-  Eigen::VectorXf normal_;
-  Eigen::VectorXf singular_values_;
+  Eigen::Vector3f normal_;
+  Eigen::Vector3f singular_values_;
   Eigen::Matrix3f cov_;
-  Eigen::VectorXf pc_mean_;
+  Eigen::Vector3f pc_mean_;
 
   vector<double> min_ranges_;
   vector<double> sector_sizes_;