Clarify the use of the "constant" attribute.

If a Variable object is set to constant, this is now meant to have the intuitive meaning: The values stored do not change during a run (a code generation target could use this for optimizations). Only AttributeVariable objects that are non-constant are updated in the namespace at every timestep (and this will be probably removed in the long term, the only current use case is the `t` of the clock and this may well be handled with a specific solution). There's a new attribute `constant_size` for `DynamicArrayVariable`, stating whether the size of the array can change during a run (e.g. in monitors), this is necessary to update the reference to the underlying array.

There's still one inefficiency here: The reference to the underlying array is never actually used in the case of monitors, so we wouldn't necessarily need to update the reference. I think -- as we discussed earlier -- we should probably make clear for code whether it only reads from dynamic arrays (e.g. synaptic updates) or only writes to them (e.g. monitors).

Ready to merge from my side for this smallish change, only in case travis does not disagree of course ;)