Passing arguments to activities as dictionaries instead of as list of lists.

# Feature
## Desired Behavior / Functionality

At the moment, when you register an activity ~~in the Sentinel 2 notebook or in the activity tests~~, if you want to return an argument or modify data, you need to pass it as input as a list or a list of lists. It would be nice to edit the code to take as input dictionaries, making the code much more readable and understandable.

Furthermore, it would be nice to have a dedicated mechanism to access values return from activities.

## What Needs to Be Done
- Change args for activities to be kwargs
- Process return values of activity functions in some way
- Update Sentinel 2 example notebook from #38 to use this

## How Can It Be Tested
The outputs should be the same as now.