Add DMPlexIsDistributed().

• Fix #388 (closed).
• One of MRs created by splitting !1949 (closed).
• Let's do this as simple as possible for now, although with allreduce in every call.
  • We are not yet ready to base this on presence of PointSF, and to stash the flag neither.
  • Harder than expected and not that much needed as of now, I guess.
  • But I need DMPlexIsDistributed() itself ASAP.
  • I will create an issue instead.
    • Related thread !1949 (comment 211564995)

TODOs:

• create the issue
• rebase onto master once !2060 (closed) gets merged in (no longer dependent on !2060 (closed))
• delete the temporary base branch haplav/temp-base/feature-dmplexisdistributed