Set truncated flag before callback invocation

[sidnarayanan]

In these lines, we mark a trajectory as truncated after callback.after_transition() is invoked, so the callback cannot tell if this transition is the last of the rollout. We should flip the order:

ldp/ldp/alg/rollout.py

Lines 241 to 252 in 1bc0330

	await store_step(step)
	# set things up for the next iteration
	agent_state = step.next_agent_state
	obs = step.next_observation
	if step.done:
	break
	if timestep + 1 == max_steps:
	trajectory.steps[-1].truncated = True
	break

[sidnarayanan]

Note to self: #118 did not add a unit test