Enable to tag ReactionRules in a Model

[kaizu]

For grouping ReactionRules in a Model, the Model class should provide the interface to tag them as something like the followings:

with reaction_rules():
    A > B | 1 | _tag('upstream')
    B > C + D | 2 | _tag('upstream')
    C > ~C | 3 | _tag('downstream', 'degradation')
    D > E | 4 | _tag('downstream')
    E > ~E | 5 | _tag('downstream', 'degradation')

m = get_model()
print(len(m.get_reaction_rules_by_tag('upstream')))  # => 2

[kaizu]

with reaction_rules_with_tag():
    A > B | 1 | _tag(upstream)
    B > C + D | _tag(upstream)
    C > ~C | 3 | _tag(downstream, degradation)
    D > E | 4 | _tag(downstream)
    E > ~E | 5 | _tag(downstream, degradation)