Layer weights parameter for the find_partition_multiplex function

[bfarahdel]

A layer weights parameter has been added to the find_partition_multiplex function. By default, a layer weight of 1 will be assigned to each layer if the layer weights are not specified by the user as done in the original implementation. In this new implementation, the user can assign weights for each layer for multiplex community detection, which can be beneficial in cases when the layer weights would not equal 1 (e.g. negative links).

#128

[vtraag]

Thanks, this can indeed be useful to some people. Merging it.