Stand alone CUDA compilation for threshold_watcher in gpu backend (#345)

Refactor the threshold_watcher and stack data structures in the gpu backend so that they are amenable to separable compilation.

* Make `gpu::stack<T>` have a host-only interface that wraps a POD type `gpu::stack_base<T>`.
* Implement a `push_back(stack_base, value)` method in `backends/gpu/kernels/stack.hpp` that is visible only to device code.
* Move `test_thresholds` kernel to a .cu file, replacing template parameters with types provided by `backends/fvm_types.hpp`.
* Add a simple C function interface, callable from host side code, defined in `backends/gpu/threshold_common.hpp`.
* Simplify the `gpu::impl::padded_size` function (both to read and in terms of efficiency).
* Use `typeid` as the default for pretty-printing types in the memory back end.
* Update the `test_gpu_stack` unit test to support new gpu stack interface.
* Fix bug in the `test_spikes` unit test, which was not running the GPU back end in the cuda unit tests.