Retrieve properly the activity for exps linked to multiple ids

[sloosvel]

Fixes #248

[jvegreg]

There was a problem with the test, as we are now returing a string and not a list of strings. I fixed it.

It should not be a problem for the datafinder, as tags can be both

[zklaus]

I couldn't test yet, but just to clarify, @jvegasbsc: There are two ways lists could enter here:

• One experiment has two activities associated. In this case this PR correctly uses only the first one.
• One dataset spans two experiments, given as a list in {dataset: ..., exp: [historical, ssp585]}. In this case there will also be two activities, one for every experiment. Is this second case considered?

[sloosvel]

* One dataset spans two experiments, given as a list in `{dataset: ..., exp: [historical, ssp585]}`. In this case there will also be two activities, one for every experiment. Is this second case considered?

Yes, this is also considered. If the experiment is a list, the code loops over experiments. And for each experiment, if it is associated to multiple activities it only considers the first one.

The test needed to be fixed because if there is only one experiment, what gets returned is just the value of the activity, instead of list with one value. But if there are multiple exps, the activity is returned as a list as well. At least that's what I get when I try it.

[mattiarighi]

@axel-lauer does this solve the problem you mentioned last week?

[zklaus]

Works nicely!