Add out-of-place implementations

[jpsamaroo]

Currently all neurons/synapses are implemented as in-place operations using nested for loops. While this is great for single-CPU performance, it has a number of downsides:

• We can't automatically use BLAS or sparse BLAS routines when available.
• We can't use AD libraries like Zygote as we mutate arrays (which most AD libraries don't support easily, performantly, or at all).
• To run on GPUs, we'd either need to switch to using GPUifyLoops.jl (which isn't a terrible idea), or write custom kernels (not difficult, but duplicative, especially if supporting both CUDA and AMDGPU, which I fully intend to implement).

An OOP implementation using broadcasting should solve many of these problems. I propose implementing these in parallel to our current implementations, using dispatch or kwargs to choose which to use at runtime.