Changes in DART group/team management

[devreal]

Changed the type of dart_group_t to be a pointer typedef.

The interface handles group allocation now similar to MPI: a group OUT parameter of a function is allocated by the function, the user does not have to provide allocated group objects for them. However, the user is responsible for freeing the group objects after use. The only use case for dart_group_create now is to create an empty group.

I also implemented clone functions for group and team objects. However, I currently see no use case for explicit assign functions as the basic types dart_group_t and dart_team_t can be assigned and invalidated using meaningful defaults by the user.

Please review carefully and suggest any changes you would like to see.

Addresses #167