add interface for defining 64-bit global ids

Internally global ids are represented using the xt_int datatype, which is an integer type provided by yaxt. During configuration of yaxt, the number of bits for this type can be specified (8, 16, 32, 64, 128 bits). Hence, YAC should be able to support global grids with more than 2^32 cells/vertices/edges.

However, the YAC interface for defining global ids currently accepts only int, which are then converted to xt_int. Another issue is the count argument of various MPI-routines, which is of type int. It needs to be checked whether this cases an error with very high resolution grids.

• add C/Fortran global id definition interface that accepts xt_int
• test configuration that uses global grids with more than 2^32 cells/vertices/edges
• test configuration where a single process has more than 2^32 cells/vertices/edges