Suggestion: streamline demos with "default" locations for downloaded data

[stevevanhooser]

You could have users put their sample data somewhere specific in the demos, so that the user (who might be quite a novice) doesn't have to make choices.

Ideas:

• [userpath filesep 'Documents' filesep 'nptk' filesep 'sampleData']
• [userpath filesep 'Documents' filesep 'nptk' filesep 'sampleOutput']

or maybe [toolboxpath filesep 'sampleData'] (where toolboxpath is the toolbox path)

You can see we make a toolboxpath function in DeepInterpolation: https://github.com/MATLAB-Community-Toolboxes-at-INCF/DeepInterpolation-MATLAB/blob/main/%2Bdeepinterp/toolboxpath.m