Use explicit init.comm in examples

This should let "make check" pass with --disable-default-comm
eventually.  For now, we break when we get to the adaptivity examples
because there's nothing with a Communicator initializing the
ErrorEstimator subclasses.  I think I want to fix that one with some
compatibility work on ErrorEstimator, though, not by requiring more
changes to user code.
examples fix for --disable-default-comm-world
Fix another CommWorld-using example
Fix accidental omission of fem_system_ex2 Mesh dim
Example SumShellMatrix creation should use a comm

This fixes miscellaneous_ex4 in the --disable-default-comm-world case
Add communicator for miscellaneous_ex6 SerialMesh
miscellaneous_ex7 --disable-default-comm-world fix
reduced_basis --disable-default-comm-world fixes
Add and use LibMeshInit::communicator() accessor
Merge pull request #79 from roystgnr/examples_fixes

Use explicit init.communicator() in examples