Fixed overwriting of multiple annotations for the same item in analogsignals and spiketrains
changed pynn uS from 1e-9*S to 10e-6*S
changed pynn uS from 10e-6*S to 1e-6*S
Tags (=manual events) in ABF is somethings  that should be attached
 to Block but imcompatible with neo (because attached to segment).
This is now attached to the first Segment of the Block.
use modified matplotlib .gitignore for more reliably ignoring of files
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