Standalone read variables

With some relatively small changes, state variables (including monitored variables) can now be retrieved using standard Brian syntax in standalone scripts (after a simulation has been run with `run_project`). I did not bother with any caching, repeatedly accessing state variables will repeatedly read from disk. I don't think that makes any noticeable difference most of the time, but it is clearly sub-optimal, in particular for things like synaptic indices that are used repeatedly. Also, no write to synaptic variables after a run is prevented, this will give odd results because a later read will not take these changes into account. But writing to variables after a run was also possible before the changes in this branch.

I updated the tests, examples and docs -- from my side this is ready to merge.

Closes #177