Adds seeding mechanism for StimulusGroup
Fixes a problem with the random number generation
Fixes a bug in StimulusGroup

Inproper seeding lead to problems here.
Initial commit of new WeightPatternMonitor

This monitor can load a pat file via load_patterns(string) and
then monitors in regular intervals the mean connection strenght
within (and between the assemblies) which are loaded.
Adds monitoring for asymmetric patterns to WeightPatternMonitor
Implements quick and dirty fix for StimulusGroup

If now stimuli are loaded into StimulusGroup it used to write a line
into the time series (stim) file every timestep. This fix resolves
that issue in a not very elegant way and forces the write to occur
every 10s of simtime.