Class DoubleRect2dSwitchboard

Special 2d Switchboard where each inner point is covered twice.

First the input is covered with non-overlapping rectangular fields.
Then the input is covered with fields of the same size that are shifted
in the x and y direction by half the field size (we call this the
uneven fields).

Note that the output of this switchboard cannot be interpreted as
a rectangular grid, because the short rows are shifted. Instead it is
a rhombic grid (it is not a hexagonal grid because the distances of the
field centers do not satisfy the necessary relation).
See http://en.wikipedia.org/wiki/Lattice_(group)

Example for a 6x4 input and a field size of 2 in both directions:

long row fields:

1 1 2 2 3 3
1 1 2 2 3 3
4 4 5 5 6 6
4 4 5 5 6 6

short row fields:

* * * * * *
* 7 7 8 8 *
* 7 7 8 8 *
* * * * * *

Note that the short row channels come after all the long row connections in
the connections sequence.

public attributes (in addition to init arguments and inherited attributes):
    unused_channels_xy
    long_out_channels_xy -- Output channels in the long rows.

Calculate the connections.

Keyword arguments:
in_channels_xy -- 2-Tuple with number of input channels in the x- and
    y-direction (or a single number for both). This has to be
    specified, since the actual input is only one 1d array.
field_channels_xy -- 2-Tuple with number of channels in each field in
    the x- and y-direction (or a single number for both).
    Must be even numbers.
in_channel_dim -- Number of connections per input channel
ignore_cover -- Boolean value defines if an
    Rectangular2dSwitchboardException is raised when the fields do not
    cover all input channels. Set this to True if you are willing to
    risk loosing input channels at the border.