refactor output

[inimaz]

Goal of this PR

Refactor output so it is easier to add new output methods.

Before this PR

• output.py file contains all the methods to output the emissions data.
• prometheus folder contains the files to initialize the PrometheousOutput

  
  ...
  ├── output.py
  ├── prometheus
  │   ├── __init__.py
  │   ├── metric_definitions.py
  │   ├── prometheus.py

# After this PR

... ├── output.py ├── output_methods │ ├── init.py │ ├── base_output.py │ ├── emissions_data.py │ ├── file.py │ ├── http.py │ ├── logger.py │ └── metrics │ ├── init.py │ ├── metric_docs.py <==== contains metrics docs to be reused if we want to create other metrics (e.g. opentelemetry) │ └── prometheus │ ├── init.py │ ├── metrics.py │ └── prometheus.py

* The output methods are split by type (file output, output to logger, http, metrics...)
* `output.py` is only to simplify imports (we can remove it if needed)