Refactor from `snappy` to `snapista`

[loreabad6]

As mentioned in this forum entry, snappy is currently not recommended for a SNAP and Python implementation by SNAP developers due to performance issues. Current solution is to use snapista.

For now, since this is work in progress, I will stick to snappy, given the large number of tools and code available to get started, but it might be smart to migrate at some point.

[loreabad6]

Although it seems he best long-term solution, I will keep this on hold for following implementations of the workflow.