Bug in mpi_get from api.module

When using mpi_get with global numbering, if a node is on the interface of the partitioned mesh, the output of the function seems to be wrong for the given node. I suggest that as _mpi_get_array_function, mpi_get function should " For all interface node only keep the value for the node with the lower rank".

Conducted tests: On the estimation test case from T2D with i=153 as an observation point and 3 processors mpi_get() and mgi_get_array()[i] returns different values.