Only save combined data_type in only-combining mode

[dachengx]

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

Close #876.

The reason for this PR is in the issue description.

Can you briefly describe how it works?

If _combining_subruns is True, only save the targeted data_type.

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: 91.236% (+0.1%) from 91.118%
when pulling 84e7697 on savers_combining_subruns
into 41a21bc on master.