Select targeted software frontend in a clever way

[dachengx]

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

Copy the codes from strax.Context.copy_to_frontend to strax.Context._get_target_sf and strax.Context._check_merge_per_chunk_storage_kwargs and use then in strax.Context.merge_per_chunk_storage.

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: 90.342% (+0.02%) from 90.327%
when pulling 9791b43 on better_merge_per_chunk_storage
into 0696ef5 on master.