ENH: support 'transform' specification for geometric decomposition

- can be used for block-like meshes that are not aligned with the global
  coordinate directions. Alternatively, for general testing purposes.

  Example,

    method  simple;
    coeffs
    {
        n       ( 2 2 2 );
        transform
        {
            origin  (-0.15 0.15 0);
            e1      (1 1 0);
            e3      (0 0 1);
        }
    }