Use parentheses to separate the class name and attributes in the representation of StorageFrontend

[dachengx]

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

Previously the representation of StorageFrontend was somehow confusing:

[straxen.storage.rundb.RunDB, readonly: True, strax.storage.files.DataDirectory, readonly: True, path: /dali/lgrandi/xenonnt/raw, take_only: ('raw_records', 'raw_records_he', 'raw_records_aqmon', 'raw_records_nv', 'raw_records_aqmon_nv', 'raw_records_aux_mv', 'raw_records_mv'), strax.storage.files.DataDirectory, readonly: True, path: /dali/lgrandi/xenonnt/processed, strax.storage.files.DataDirectory, readonly: True, path: /project2/lgrandi/xenonnt/processed]

This PR use the parentheses to separate the class name and its attributes:

[straxen.storage.rundb.RunDB (readonly: True), strax.storage.files.DataDirectory (readonly: True, path: /dali/lgrandi/xenonnt/raw, take_only: ('raw_records', 'raw_records_he', 'raw_records_aqmon', 'raw_records_nv', 'raw_records_aqmon_nv', 'raw_records_aux_mv', 'raw_records_mv')), strax.storage.files.DataDirectory (readonly: True, path: /dali/lgrandi/xenonnt/processed)]

Can you briefly describe how it works?

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

[github-actions]

coverage: 91.506% (+0.09%) from 91.42%
when pulling 56a0e81 on sf_repr
into 55d82b0 on master.

[coveralls]

coverage: 91.47% (+0.05%) from 91.42%
when pulling 56a0e81 on sf_repr
into 55d82b0 on master.

[yuema137]

@dachengx I checked that the representation of StorageFrontend is not used directly and is just a property to provide information. So the format change looks good to me.