Feature request: Random stationary mobility with constrains

[esterl]

Add the possibility of adding some constrains to the Stationary mobility model, e.g. establish a min and max distance between a pair of nodes. That way, one could ensure that the resulting network is connected.

Other possible constrains could be:

• The max number of nodes within another node's radius (to limit the network density)
• The "algorithm" to grow the network, e.g. one could use preferential attachment, so that when adding a new node, the probability of choosing a previous node to be close to (within that max distance) is derived by the number of connected nodes. Something like "graph generating algorithms".