Upgrades to Nets

[lorenzori]

updated the NN feature extractors for both sentinel and google. Just making the codebase more elegant. Couple of important points:

• Upgraded to Python 3.6.6 (from 3.5) and to Tensorflow 1.12 (from 1.4)
• Google data source accepts the step parameter, but we only takes the 8 images around the one of the cluster centre max.
• we call the layer from the net that extracts the features by name and dont pop stuff out anymore.
• fixed tests to work on Travis.