Option to choose how to create bus_regions and cluster

[davide-f]

In the new version of the code with the alternative clustering developed in #134 there are two options
a) The typical version where bus_regions are created according to the voronoi cells and cluster_network is performed using kmeans algorithm
b) a custom shape is provided to cluster the network. In this case, the bus_regions are loaded from gadm and the cluster_network use the shapes of gadm as well.

The latter approach could be decomposed in two independent features:

1. in bus_regions a user can choose between creating the bus_regions according to the voronoi cells or custom shapes, such as gadm
2. in cluster_network, the user could choose between gathering the substations using kmeans or optional shapes

Currently, option 1 and 2 are hard coded so that the user cannot choose hybrid/mixed approaches between (a) and (b)

[davide-f]

Old issue duplicate of the more recent #486