Fixes #482 - Handles timestamps of StepCurrentSource for NEST

[appukuttan-shailesh]

Continuing on the discussion under issue #482 :
The sequence of steps for NEST is now as follows:
(consider dt = min_delay = 0.1 ms)

1. User inputs a time series, e.g.
   times = [0.0, 0.1, 0.2, 0.3, 0.4, 0.5]
2. _delay_correction() will subtract min_delay (set to 0.0, if result is negative), so
   times = [0.0, 0.0, 0.1, 0.2, 0.3, 0.4]
3. We now scan the time series to find the last element matching dt. This corresponds to the first timestamp that can be used by NEST for current injection. All prior timestamps (and corresponding amplitudes) are removed. So we have
   times = [0.1, 0.2, 0.3, 0.4]

A thorough test case for the StepCurrentSource across all simulators will be developed for issue #512 , and thus the same is avoided here.

The fix has been tested for the issue reported for the Izhikevich model under OpenSourceBrain (see here) and found to resolve the same.

[coveralls]

Coverage decreased (-0.02%) to 54.162% when pulling 0fb6fe1 on appukuttan-shailesh:fixes_482 into ad00b7a on NeuralEnsemble:master.

[coveralls]

Coverage increased (+0.02%) to 54.209% when pulling 75d3a6b on appukuttan-shailesh:fixes_482 into ad00b7a on NeuralEnsemble:master.