Make the direct use of `TimedArrays` in Python consistent with the interpretation during simulation

Small fix, now also uses the "upsampling" technique when directly calling a `TimedArray` from Python (which is something one would only do for interactive use or debugging, I think). There are some weird corner cases where this will not necessarily give consistent answers when the group using the `TimedArray` uses a dt that is much smaller than `defaulclock.dt`, but there's not really anything we can do about it.

I also added a test that explicitly checks for the `[t, t+dt)` semantics (as opposed to the `[t-dt/2, t+dt/2)` semantics in Brian 1).

Ready to merge from my side.