Revamp the attribute system

[JTS22]

This PR introduces a new attribute system to DPsim. Attributes are variable properties that can hold some numerical value, which can then be accessed from the outside by the simulation scheduler or by other components. Attributes can be computed from other attributes, therefore forming a dependency graph which is used in the scheduler to determine the execution order of tasks.

With this PR, every attribute can either be static or dynamic. Static attributes do not have any immediate dependencies, while dynamic attributes can be entirely derived from other attributes. For more details on the distinction between dynamic and static attributes, see this comment in issue #61

The usage of attributes in the component code remains roughly the same, however there are some differences:

• Attributes are public member variables of the components they belong to and they usually have type const Attribute<T>::Ptr.
• Attributes are created in the constructors initialization list. A new attribute can be created with the Attribute<T>::create and Attribute<T>::createDynamic methods.
• To acquire a (mutable) reference to an attribute's underlying data, the dereference operator has to be used twice on the attribute member variable, resulting in **mAttribute.
• Derived attributes can be created using the various derive-functions on the attribute object.