minor change to null.py, adding timeseries

adding generators/timeseries.py which models signals and compositions of signals for use in testing the time series detection capabilities of the tools.
Added tests for timeseries + other modifications

Added tests for timeseries and modified the timeseries.py generator to eliminate some redundancy and reduce the number of functions. Also changed the way functions were being called.
Merge branches 'timeseries_generator' and 'master'