Tpetra::Map/Epetra_BlockMap: preserve indexBase in ReplaceCommWithSubset

Created by: mhoemmen

@trilinos/xpetra @trilinos/tpetra @tawiesn @csiefer2

The index base is redundant; users can get this information from the min or min overall global index. Furthermore, it's confusing that Epetra requires the index base to equal the min overall global index. This causes problems with implementing subset and strided Maps.

Changing this requirement on the index base might break backwards compatibility. The easiest way to work around for now would be for SameAs to ignore the index base.