introduces two new methods interlace and deinterlace to 'interlace' resp. 'deinterlace' blockvectorsi. Implemented so far as
interlace: \f$\left((R^k)^m\right)^n\longrightarrow(R^k\cdot n)^m\f$, i.e. for example
      / / /a11\ \ \               / /a11\ \
     | |  \a12/  | |             | | a12 | |
     | |  /a21\  | |             | | b11 | |
     | |  \a22/  | |             | | b12 | |
     | |  /a31\  | |             | | c11 | |
     |  \ \a32/ /  |             | | c12 | |
     |             |             | | d11 | |
     |  / /b11\ \  |             |  \d12/  |
     | |  \b12/  | |             |         |
     | |  /b21\  | |             |  /a21\  |
     | |  \b22/  | |             | | a22 | |
     | |  /b31\  | |             | | b21 | |
     |  \ \b32/ /  |  interlace  | | b22 | |
     |             | ----------> | | c11 | |
     |  / /c11\ \  |             | | c22 | |
     | |  \c12/  | |             | | d21 | |
     | |  /c21\  | |             |  \d22/  |
     | |  \c22/  | |             |         |
     | |  /c31\  | |             |  /a31\  |
     |  \ \c32/ /  |             | | a32 | |
     |             |             | | b31 | |
     |  / /d11\ \  |             | | b32 | |
     | |  \d12/  | |             | | c31 | |
     | |  /d21\  | |             | | c32 | |
     | |  \d22/  | |             | | d31 | |
     | |  /d31\  | |             |  \d32/  |
      \ \ \d32/ / /               \       /

[[Imported from SVN: r6541]]