fvec supported: create a dense block matrix
load tested:
  a2.txt  a.txt  b.txt  c.txt  d2.txt  f.txt
  g2.txt  g.txt  h.txt

fvec debuged:
  Eigen::MatrixXd and Eigen::VectorXd

set default blk_sz 1 for linesplit and fvec:
  in partition.hpp, scheduler.hpp and loader.hpp