Revise models of internal gains subpackage

What is the problem?

Most models in subpackage AixLib.Utilities.Sources.InternalGains have an input called schedule.
This input is actually limited to values between 0 and 1. But this is not asserted.

Additionally, the Humans models do not extend from the PartialInternalGain model.

How do we want to solve it? Describe the solution you'd like

1. Refactor the input connector Schedule in AixLib.Utilities.Sources.InternalGains.BaseClasses.PartialInternalGain to schedule.
2. Let the Human models extend from the PartialInternalGain model.
3. Add assertion statement in the partial model PartialInternalGain.