reduce size of example data files

[TomDonoghue]

The data files in examples/data/ are pretty big files (~47 MB each). These big files make the repo needlessly big to clone / download, etc.

From a quick check, we only use these files once, in the plot_theta_feature_distributions example. Within that example, we immediately extract 1 second of data (about 1% of the length), so most of this data isn't even used in the example.

Basically, we should trim and replace the data files and only package the 1 second segments in examples/data/.

@ryanhammonds - can you double check if these files are used anywhere else? If not, please make the minimized data files, and replace in examples.

Sidenote: I wouldn't be opposed to keeping a collection of some bigger datafiles that are used for examples / testing, etc - but if so, these could be put in a separate place / repo (bycycle-tools/Data, for example), to still keep the code repo as small.

[ryanhammonds]

Complete in #82.