Class DoubleRhomb2dSwitchboard

Rectangular lattice switchboard covering a rhombic lattice.

All inner points of the rhombic lattice are covered twice. The rectangular
fields are rotated by 45 degree.

We assume that both the first and last row is a long row, e.g.

*   *   *   *
  *   *   *
*   *   *   *
  *   *   *
*   *   *   *

The incoming data is expected to contain the long rows first, then
the short rows.

The alignment of the first field is chosen to minimize cutaway.

public attributes (in addition to init arguments and inherited attributes):
    out_channels_xy

Calculate the connections.

Note that the incoming data will be interpreted as a rhombic grid,
as it is produced by DoubleRect2dSwitchboard.

Keyword arguments:
long_in_channels_xy -- 2-Tuple with number of long input channels in
    the x- and y-direction (or a single number for both).
diag_field_channels -- Field edge size (before the rotation).
in_channel_dim -- Number of connections per input channel