Create `neo.units` module

[apdavison]

As a way to start addressing #133 I suggest creating a module neo.units, which for the time being would just import all of the units from quantities, but would make it easier in future to move to pint or other library since we would avoid using the quantities (or pq) namespace.

All of the documentation, examples and docstrings should also be updated to use the new module.

[zm711]

@apdavison, now that you are working on quantities do you still want this on the list of to-dos?

[apdavison]

I think it's still worth doing for long-term maintainability, but it's not urgent.

[apdavison]

quantities seems to be working ok, so let's close this for now. We can always re-open this later.