Refactor

[lorenzori]

first step of the refactoring initiative. Following changes:

• data_source base class
• sentinel and google images classes that inherit from data_source and are only dependent on coordinates, dates etc (no more GRID)
• starting to cover the code with unit-tests and integration with Travis CI (WIP)
• S3 buckets for the google and sentinel NN models.
• moving keys and co. to env variables for simplicity

[pasquierjb]

Good job