ExternalCode needs method to access allocated hosts

[fzahle]

Realised that the actual ExternalCode instance needs a way to get hold of the resource allocated to the case by the CaseIterator. Rather than having the MPI_Allocator reserve the CPUs through its deploy() function, it should simply just return an ObjectServer and let the ExternalCode call get_hostnames, which will reserve the hosts. Guess the allocator's release function should still be responsible for the release of the CPUs.