Rename a few local variables to avoid ambiguity with member variables.

Rename a few local variables to avoid ambiguity with member variables.

The `MGLevelGlobalTransfer` class has member variables `copy_indices_*` but in the
`fill_and_communicate_copy_indices()` member function also allocates local variables
of the same name. Rename these to make the distinction clearer.

While there, also initialize a few member variables in the constructor of a local
class.