Get the weight matrix between two sets of units
SnnsR low-level function to get the weight matrix between two sets of units.
# S4 method for SnnsR getWeightMatrix(unitsSource, unitsTarget, setDimNames)
a vector with numbers identifying the source units
a vector with numbers identifying the target units
indicates, whether names of units are extracted and set as row/col names in the weight matrix
the weight matrix between the two sets of neurons