Do not render the template without arguments to analyze its content (for USES_VARIABLES and ITERATE_ALL) -- instead directly use the template source. This removes the need for the `{% if ... is defined %}` blocks in the templates.
Remove the now unnecessary `{% if ... defined %}` blocks.
Make `__setitem__` delegate to a `set_item` method with an additional `level` argument -- this way it can also be called directly if necessary (e.g. in `PoissonGroup`).
(Re-)implement setting of variables via arrays in standalone. Doesn't cover subgroups and needs more tests, though. Closes #192
Don't do the "take all frames *starting* from the calling frame" into account (i.e., revert commit fb79a66bd). This led to all kind of warnings.
Allow accessing indices via `group.indices[...]`. Closes #176