Added OECD Weekly Tracker of GDP growth

Today the OECD Weekly Tracker of GDP growth was added to DBnomics.

It provides a real-time high-frequency indicator of economic activity using machine learning and Google Trends data. Please read its documentation for more details.

The dataset is available on DBnomics as OECD/GDP_GROWTH.
It has also been added to the OECD category tree.

On the technical side, this dataset is distributed on its own GitHub repository in clean Excel files. It’s not available on the OECD SDMX API, so we had to write a specific (and relatively simple) download and conversion code to integrate it.

Note: here is the related issue and merge request on the oecd-fetcher project.

:tada: