Making next_free_pair_object and next_free_single_object as inline functions

Making next_free_pair_object and next_free_single_object as inline functions

Also contains a patch from Wolfgang with changes to the instantiation file.

Background: In release mode, GNU-4.8.x (OSX) seem to optimize away the calls when
specializing the template functions and then symbol definitions seem to be different
between debug and optimized modes. This causes undefined linkage for release mode.

References:
[1] https://groups.google.com/forum/#!topic/dealii/c2qjAsuJ7Mc
[2] https://code.google.com/p/dealii/issues/detail?id=137
[3] https://groups.google.com/forum/#!msg/dealii/6RXRR-iac0Y/Tjzwg1rFRMQJ
This function should be declared as 'inline'. If it gets used by more than one compilation unit, it will lead to duplicate symbols otherwise.