Merge mask in Combiner.sigma_clipping, instead of overwriting it

[kbwestfall]

As titled. Addresses #849 .

---

• For new contributors: Did you add yourself to the "Authors.rst" file?

For documentation changes:

• For documentation changes: Does your commit message include a "[skip ci]"?
  Note that it should not if you changed any examples!

For bugfixes:

• Did you add an entry to the "Changes.rst" file?
• Did you add a regression test?
• Does the commit message include a "Fixes #issue_number" (replace "issue_number").
• Does this PR add, rename, move or remove any existing functions or parameters?

For new functionality:

• Did you add an entry to the "Changes.rst" file?
• Did you include a meaningful docstring with Parameters, Returns and Examples?
• Does the commit message include a "Fixes #issue_number" (replace "issue_number").
• Did you include tests for the new functionality?
• Does this PR add, rename, move or remove any existing functions or parameters?

Please note that the last point is not a requirement. It is meant as a check if
the pull request potentially breaks backwards-compatibility.

---

[kbwestfall]

Closing for now, while I make additional edits.