Tidy up Modelica.Magnetic.FluxTubes.Material.SoftMaterial ... #4799

... to make the directory more organized:
Create a subdirectory "Roschke" and move the following files:

• mu_rApprox -> Roschke.Functions.mu_rApprox
• app_mu_r -> Roschke.Functions.app_mu_r
• BaseData -> Roschke.BaseData
• ElectricSheet.* -> Roschke.ElectricSheet.*
• PureIron.* -> Roschke.PureIron.*
• CobaltIron.* -> Roschke.CobaltIron.*
• NickelIron.* -> Roschke.NickelIron.*
• Steel.* -> Roschke.Steel.*
  This way it is much easier to find material records belongig to different approximation types.
  Since this is not backwards compatible, this should be done when a conversion script is allowed (major release).