Fixed Nengo compatibility

[bmorcos]

Looks like the learning_rate attribute is now read-only which broke the --adapt code:

Traceback (most recent call last):
  File "adaptive_bias.py", line 139, in <module>
    AdaptiveBias().run()
  File "c:\users\user\documents\github\ctn_benchmarks\ctn_benchmark\benchmark.py", line 95, in run
    model = self.model(p)
  File "adaptive_bias.py", line 81, in model
    conn.learning_rule_type.learning_rate *= p.learning_rate
  File "c:\users\user\documents\github\nengo\nengo\params.py", line 205, in __set__
    super(NumberParam, self).__set__(instance, value)
  File "c:\users\user\documents\github\nengo\nengo\params.py", line 97, in __set__
    self.validate(instance, value)
  File "c:\users\user\documents\github\nengo\nengo\params.py", line 226, in validate
    super(NumberParam, self).validate(instance, num)
  File "c:\users\user\documents\github\nengo\nengo\params.py", line 146, in validate
    raise ReadonlyError(attr=self.name, obj=instance)
nengo.exceptions.ReadonlyError: PES.learning_rate: learning_rate is read-only and cannot be changed

I also formatted for pep8 which accounts for the majority of the diff.

The functional change is on line 86 where I hardcoded the default PES.learning_rate=1e-4 * the scale factor.