Checking if responses exist for realization is very slow

[yngve-sk]

Edit: It is faster now, more info in #8369

Because it checks via file system, loading and checking the files (xr datasets mostly) again and again. the bigger the files, the slower the loading time. In one really big case it takes 25 minutes to open the ert gui;

This blocks the combined datasets PR from being merged. #8247

[oyvindeide]

Blocked by: #8369

[oyvindeide]

Related to: #8369, will close it though, as it is currently much faster than it was.