Serialization support for KeyValueTree

Serialization support for KeyValueTree

Implement serialization of KeyValueTree objects to xdr files for use in
storing inputrec values using this format.

There is a C++ ISerializer interface in between to support testing, but
in the future, this could possibly be replaced with more direct xdr
serialization.  However, our current gmx_system_xdr.cpp does not support
in-memory serialization, which would be a requirement for broadcasting
the data structure, so the ISerializer can provide a useful abstraction
also for this.

Change-Id: I6836d8e7f725c364336e09745052521eccf331ce