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)

[benoit-cty]

Nice organization, thanks !