Add representation to strax.StorageFrontend

[JoranAngevaare]

What is the problem / what does the code in this PR do
This fixes #332

Can you briefly describe how it works?
It allows you to print the StorageFrontend Class providing you with more usefull info.

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

st = straxen.contexts.xenonnt_online()
for sf in st.storage:
    print(sf)

returns

straxen.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_mv')
strax.storage.files.DataDirectory, readonly: True, path: /dali/lgrandi/xenonnt/processed
strax.storage.files.DataDirectory, path: ./strax_data

This used to return:

<straxen.rundb.RunDB object at 0x7fa902955580>
<strax.storage.files.DataDirectory object at 0x7fa902955fd0>
<strax.storage.files.DataDirectory object at 0x7fa902955910>
<strax.storage.files.DataDirectory object at 0x7fa9029f4bb0>

[darrylmasson]

Do we maybe also want to add a __str__ that returns the same? Might prevent some posts in tech support later.