Enable automatic downloading of data examples 

We should create a small program that evaluates whether or not you've got the necessary files to run an example script in [echopy/tests](https://github.com/open-ocean-sounding/echopy/tree/master/tests). If not, they will be downloaded to [echopy/data](https://github.com/open-ocean-sounding/echopy/tree/master/data) after asking for permission. 