ENH: warn instead of raise violated rules

[redeboer]

So far, when there are violated rules and/or non-executed rules, find_solutions() raises these as a RuntimeWarning. This is an incorrect use of this exception class that also prevents the user from inspecting the execution info. The RuntimeWarning is therefore now passed through warnings.warn(). The warnings can then be switch off with:

import warnings
warnings.simplefilter("ignore", category=RuntimeWarning)

Still, if there are violated rules, there are no solutions, so an exception is still raised (now as a RuntimeError instead of a ValueError. ExecutionInfo is now passed on to this exception, so that it can be further inspected:

import qrules
try:
    reaction = qrules.generate_transitions(
        initial_state=["pi0"],
        final_state=["gamma", "gamma", "gamma"],
        allowed_interaction_types=["em"],
    )
except RuntimeError as e:
    msg, execution_info = e.args
execution_info.not_executed_node_rules

See also #174