Reduce the number of time-series displayed by default

When working with connectivity matrices with a large number of nodes or in the case of surface-based simulations, the default time-series visualizers tries to display all the data (for surfaces normally more than 16k nodes) which provokes the browser to crash and the system becomes really unresponsive.

A possible solution would be to display a) an aribitrary number of time-series by specifying a number; or computing a number by specifying a percentge of the total number of nodes in the network