Separate time dependent dirichlet and subdomain deactivation

Further refactorings of the time interval Dirichlet boundary conditions and the subdomain deactivation.

• Cleanups and sorting.
• Extraction of create function in own file.
• Separation of similar but at the end different DirichletBC and DeactivatedSubdomain Dirichlet classes.
• Simplifications to TimeInterval.
• Further refactorings in the DeactivatedSubdomainDirichlet.

Review commit-wise.

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?