fix of example extension and dynamic class

firing_rate_extension.py

#fix of https://gist.github.com/nicain/be598d5b7be5f2ffe1fd7e8d27c87ec3
# 
from pynwb.spec import NWBNamespaceBuilder, NWBGroupSpec, NWBAttributeSpec
import numpy as np
from pynwb import get_class, load_namespaces

mylab = 'alleninstitute'
ns_path = "%s.namespace.yaml" % mylab
ext_source = "%s.extensions.yaml" % mylab

ns_builder = NWBNamespaceBuilder('Extension for use in my Lab', mylab)
ext = NWBGroupSpec('A custom FiringRate for my lab',
                   attributes=[NWBAttributeSpec('foo', 'float', 'name of population', required=True),
                               NWBAttributeSpec('bar', 'float', 'name of population', required=True)],
                   neurodata_type_inc='TimeSeries',
                   neurodata_type_def='FiringRateSeries')

ns_builder.add_spec(ext_source, ext)
ns_builder.export(ns_path)

ns_path = "%s.namespace.yaml" % mylab
load_namespaces(ns_path)

FiringRateSeries = get_class('FiringRateSeries', mylab)
ts = FiringRateSeries('firing_rate',
                      'None',
                      np.array([.01,.05,.1]),
                      'Hertz',
                      timestamps=np.linspace(0,1,3),
                      bar=3.14,
                      foo=2.718)