Get band mean and standard deviation for better normalization

[ptresson]

📝 Changelog

• Add get_mean_sd_by_band function in data/utils.py
• Add warning if tags are unavailable and computation does not take place

I believe it would be a good idea to check if this works properly within the malpolon environment. As mentioned IRL, I had trouble setting up an environment. In my poorly setup environment, rasterio's src.tags(band_number) returns an empty dict.
I've tested in other environments and this works properly, I never had such an issue before and do not find further hints about this bug for now. But if it works in a properly set environment, it works.

Just in case, I've added a warning in case the function does not return means and sds as its supposed to do.