Add metrics for global dynamics

+ Two metrics were presented in a recent article [1]:

• metastability
• synchrony

Having them in TVB would be a plus since it gives us a means to reproduce their work.
In addition to the input arguments shared with the other TimeSeries metrics, it would be important to have and additional argument to pass an index mask. This would enable users to select the nodes over which the metrics will be computed. These measures are normally compared when obtained globally (characterizing the whole network) or locally (characterizing a small subset of nodes like Mantini networks) .

[1] Hellyer et al (2014). Opposing Actions of Cognitive Control Networkks on global Dynamics.