Improve velocity download management

As of now, we download the velocity field on a daily basis, saving data on disk using the download date in the file name

But this is only relevant for the forecasting window of the GLORYS velocity from the [Global Ocean Physics Analysis and Forecast](https://data.marine.copernicus.eu/product/GLOBAL_ANALYSISFORECAST_PHY_001_024/description)

**In VFrecovery we shall only refresh download during the 15-21 days before present**

*Details*

According to the user manual the catalogue update goes like:
> Every day the forecast delivered the previous day is overwritten by one day of simulation and the new 10-day forecast. Weekly, the simulations are replaced by the NRT-analyses (from -192 to -24 hr) and the NRT-analysis by the best analyses (from - 360 to -192 hr). The last 11-15 days of the historical time series of simulation/analysis/forecast are overwritten every day.
> The catalogue data from 2-year before present to 15-21 days before present are not updated anymore. These data have the best quality because are all best analysis (orange bar in the pictures).

Here is the example from the user manual: