Inform users of pending change of default index merging strategies

Using the methods power or composite to construct function space basis trees without an explicit index merging strategy argument will now issue a warning. This warning informs that the default index merging strategy will change after the 2.11 release. More specifically, it will change from BlockedLexicographic to FlatLexicographic for the composite method, and from BlockedInterleaved to FlatInterleaved for the power method. The consequence is that default indexing is always flat, which is the most common form of indexing.

At the same time, we warn users of the Python interface that the default layout argument for Power bases will change from lexicographic to interleaved. With these changes, both interfaces will use the same defaults.