Using different rule activation modes at group or rule levels

[develop-cs]

Is your feature request related to a problem? Please describe

Not necessarily, just two variations of the feature described in this issue #38.

Describe the solution you'd like

The following setting rule_activation_mode: many_by_group allows Arta to be used as a simple process execution engine.

But, this flow of control is applied on all the rules of a rule set.

The idea is to be able to define the scope of different rule activation modes: at rule group and even rule levels.