Add combining into the DataKey

[dachengx]

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

To solve the caveat in #871, this PR saves the combining (True or False) in strax.DataKey to distinguish whether superrun is in only combining mode.

Also, change _combining_subruns to combining because it will be an argument frequently used.

Can you briefly describe how it works?

PR first simplifies get_components and adds another attribute combining in strax.DataKey.

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

Please find the example at test_only_combining_superruns modified by this PR.

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: 90.86% (+0.05%) from 90.812%
when pulling 92120cb on combining_into_lineage
into 5e5c620 on master.