Change behavior for adding Difference Metrics

[schmoelder]

This PR aims at restructuring the behavior for adding difference metrics to the Comparator class.

Closes #36

In practice, this could would then look something like

from CADETProcess.processModel import ComponentSystem
from CADETProcess.comparison import Comparator
from CADETProcess.comparison import AbsoluteArea
from tests.test_solution import TestSolutionIOGaussian

component_system = ComponentSystem(2)
tracer_peak = TestSolutionIOGaussian(component_system)

comparator = Comparator()

metric = AbsoluteArea(tracer_peak)
comparator.add_difference_metric(metric, 'outlet.outlet')

Note, it's not required any more to register the reference with the Comparator since this was only required for instantiating the difference metrics. This is now handled outside.

Since this would be a breaking change, I tried to add deprecation warnings s.t. we can provide backwards compatibility for some time.

To do

• Discuss design changes with @ronald-jaepel
• Add tests for Comparator
• Make a plan for deprecating such interfaces
• Update documentation