enhancement for ATMODAT4.0: validate URLs

[atmodatcode]

This is a piece of code that we could integrate in a future checker enhancement that consists of checking the validity of the metadata URLs.

`from IPython.display import display
import requests

url="https://cera-www.dkrz.de/WDCC/ui/cerasearch/entry?acronym=LambWTyRSetAndGaleDaysOverTheNo"
try:
response = requests.get(url)
except:
print("Not a valid url!")
else:
if response.status_code != 200:
print("Not a valid url!")
else:
print('url is valid!')`

[jkretz]

I set up the functionality for this check in the orcid_check branch. As long as it is not in the ATMODAT standard, I am not sure if we should merge it. To activate those checks, they have to be added to the atmodat_standard_checks.yml.