title: How to slice a matrix in Eigen by taking n rows/columns and then skipping m rows/columns
body: <p>I would like to ask a question about slicing in an efficient manner in Eigen. I would like to slice the matrix in a way that permits me to take <code>n</code> rows then skipping <code>m</code> rows and repeating the procedure till the end index. For example, if A is <code>1000 x 20</code> matrix, I would like to form the matrix B which is <code>800 x 20</code>, which contains the first 4 rows of each sequential 5 rows of A.</p>
  <p>I hope that my problem is clear.</p>
  <p>Thank you very much,
  Saddam</p>