make the sampling_period property settable in spiketrain
add sampling_period property to spike object for consistenct with spike_train
use decorators for sampling_period property in analogsignal
add spike_duration property to spiketrain.  This information is available from spike objects but not spiketrain objects
finer-grained handling of corner cases, particular None values, in properties of analogsignal, spike, and spiketrain
test analogsignal None handling