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. 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.
Merge pull request #132 from brian-team/constant_attribute

Clarify the use of the "constant" attribute.