Add std of surface air temperature

[JordiBolibar]

To be investigated if transitioning to PDDs or adding information on the intra-monthly variability of surface air temperature is better. This could easily be done by computed the standard deviation based on the daily average temperature values, to understand how much the air temperature has changed across the month, which cannot be captured by just using the mean or median value.

[khsjursen]

Currently the user downloads monthly ERA-5 Land data from the Copernicus website. To compute std of surface temperature or PDDs requires the user to download daily data. Could be done this way providing a script to extract monthly PDDs and/or std of temperature for each month, but is a bit cumbersome. Best might be if we could compute and host these variables somewhere such that they can be easily retrieved by the user.