This function is used by the connect_layers function, after all
units in a layer have been constructed, to construct the links and shared
weights between two maps, one in a layer L and another in the previous layer
L-1.
In more detail: each UNIT in map is connected to a square group of units of size
map.FilterSizeX by map.FilterSizeY in prev_map. Neighbouring units in map
have connections to neighbouring groups of units in prev_map, shifted by
map.PixelShiftX and map.PixelShiftY. Furthermore, all units
are connected to such a square group using the same set of weights.
The coordinates of the top-left of this square of units can be found by considering the coordinates
of a unit in map - say (x, y): the unit number of the top-left of the square in prev_map can
be found as x * map.PixelShiftX + y * map.PixelShiftY * map.SizeX.
If either of map.FilterSizeX or map.FilterSizeY is smaller than 0, this
indicates that the maps are to be connected fully, i.e. each unit in map
is connected to all units in prev_map.
If either of map.PixelShiftX or map.PixelShiftY is smaller than 0, this
indicates that the weights should not be shared, i.e. each unit in map
is connected to a unique group of units in prev_map using unique weights.
If map.SizeX = 1 or map.SizeY = 1, there is only one unit in that
direction which is to be connected to a number of unique regions in prev_map
using shared weights.
The function first creates a set of weights (size map.FilterSizeX times
map.FilterSizeY) using malloc_weight. These weights get a SHAREDWEIGHTWeightFlag field (see WEIGHT-flags).
Then a set of links is created between a unit in map
and a group of units in prev_map, after which each link is connected to a
weight in the weight set. This is done for all units in map. If the
weights are not shared (map.FilterSizeX < 0 or map.FilterSizeX < 0),
a new set of weights is created for each unit in map.
The created LINK structures are allocated using malloc_link and
given the correct LinkFlag (INLINK or OUTLINK, see LINK-flags).
NOTE
The link_count and weight_count variables are updated (see SHAREDNET-variables).
