[MNT] - Update example data file info & usage

[TomDonoghue]

Responds to #312

I did a quick check through for information on the data files we use, and added a description to the README file in the data folder to add the basic information there. Notably, it adds a brief description of what the data is, sampling rate, and related publication. Having moved this info, I removed a bit of info in the tutorials themselves.

Also, we have a pre-computed filtered version of the DBS data file, and I don't remember why. It's barely used in the tutorials, and where it was (instantaneous measures tutorial) there's not really any reason to, so I updated this tutorial to simply compute the filtered version. This means that data_sample_1_filt is now not used anymore. I don't think we should remove it, at least for now (if we did, anyone running an older example with load_ndsp_data that uses it would get an error, which I don't think helps anything - so I think we can consider the file deprecated, but leave it there for now.

[ryanhammonds]

Looks good. The only thing I noticed is that sample_data_1_filt.npy is no longer used anywhere, I think. So should it be dropped from data/?

Edit: nevermind! just re-read your original comment