Find the frontends which stored the targets

[dachengx]

What is the problem / what does the code in this PR do

Update _get_source_sf to get_source_sf to return a list of frontends that store the target(s).

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

Please make sure that all automated tests have passed before asking for a review (you can save the PR as a draft otherwise).

[coveralls]

coverage: 91.476% (+0.05%) from 91.43%
when pulling eaf48fd on find_sf
into 0f36687 on master.

[FaroutYLq]

Thanks for the very nice function! Below is an example I tested. Looks good to me and it will be very helpful for cleaning up data availability issue.

Test on dali login node, in 2024.01.1 container:

>>> import cutax
>>> import strax
>>> strax.__file__
'/home/yuanlq/software/strax/strax/__init__.py'
>>> st.get_source_sf('053505', 'peaklets')
[]
>>> st.is_stored('053505', 'peaklets')
False
>>> st.get_source_sf('053447', 'peaklets' )
[straxen.storage.rundb.RunDB, readonly: True]
>>> st.is_stored('053447', 'peaklets' )
True