CoreNEURON does not free GPU buffers allocated for gap junctions

[olupton]

Describe the issue
CoreNEURON does not free all GPU memory that it allocates for gap junction handling:

CoreNeuron/coreneuron/network/partrans.cpp

Lines 133 to 174 in 53b0c5f

	/// TODO: Corresponding exit data cluase for OpenACC/OpenMP is missing and hence
	/// GPU buffers are not freed.
	void nrn_partrans::gap_update_indices() {
	// Ensure index vectors, src_gather, and insrc_buf_ are on the gpu.
	if (insrcdspl_) {
	int n_insrc_buf = insrcdspl_[nrnmpi_numprocs];
	nrn_pragma_acc(enter data create(insrc_buf_ [0:n_insrc_buf]) if (corenrn_param.gpu))
	// clang-format off
	nrn_pragma_omp(target enter data map(alloc: insrc_buf_[0:n_insrc_buf])
	if(corenrn_param.gpu))
	// clang-format off
	}
	for (int tid = 0; tid < nrn_nthread; ++tid) {
	TransferThreadData& ttd = transfer_thread_data_[tid];
	size_t n_src_indices = ttd.src_indices.size();
	size_t n_src_gather = ttd.src_gather.size();
	NrnThread* nt = nrn_threads + tid;
	if (n_src_indices) {
	int* src_indices = ttd.src_indices.data();
	double* src_gather = ttd.src_gather.data();
	nrn_pragma_acc(enter data copyin(src_indices[0:n_src_indices]) if(nt->compute_gpu))
	nrn_pragma_acc(enter data create(src_gather[0:n_src_gather]) if(nt->compute_gpu))
	// clang-format off
	nrn_pragma_omp(target enter data map(to: src_indices [0:n_src_indices])
	map(alloc: src_gather[0:n_src_gather])
	if(nt->compute_gpu))
	// clang-format on
	}
	if (ttd.insrc_indices.size()) {
	int* insrc_indices = ttd.insrc_indices.data();
	size_t n_insrc_indices = ttd.insrc_indices.size();
	nrn_pragma_acc(
	enter data copyin(insrc_indices [0:n_insrc_indices]) if (nt->compute_gpu))
	// clang-format off
	nrn_pragma_omp(target enter data map(to: insrc_indices[0:n_insrc_indices])
	if(nt->compute_gpu))
	// clang-format on
	}
	}
	}

This could cause problems if we repeatedly run CoreNEURON in a single process, so we should add cleanup code.

Expected behavior
CoreNEURON should clean up after itself.