Draft: Resolve "Introduce groups and dependencies on groups"

Closes #19

Introduce groups for rules and dependencies. The discussion of the action plan for "how to implement the de-registering of rules and groups " can be found in the related Issue RL#25, the examples to elaborate on the edge cases are in the list.

Main changes:

• <dependencies> can have two parameters, one for the individual rules, another for the groups which dependents on.
• If one rule belongs to a group, it should have a <groups> tag.
• RuleHandler will capture the changes of groups and the rules inside of the groups dynamically, with the checking of graph and group relations validilty.
• Add and adapt the deregister functions for rules and groups, since if remove a rule inside a group or remove a group can affect the entire program, more constraints and conditions are included.
• Optimalize the register(), check_valid_graph() and check_valid_group(), to guarantee the failed registration would not change the previous RuleHandler.

Tests:

• test if all the groups and the rules inside the groups are registered in to the RuleHandler.graph.
• test if the program gives an error if the group(s) are not valid, and the invalid group(s) and related rule(s) will be removed from the program.
• test if the de-registration of rules and groups is correct, with or without cascade, the updates of graph, groups are valid.
• test if a group where a rule is dependent on is executed before the rule itself.
• test if it works with multiple groups.
• test if the program fails when an invalid group is introduced in the process of registration.
• test if circle dependency fails (two groups dependent on each other).

\approve @ds @laurens @pdlmora