Set unit instead of having it in description

[HansOlsson]

Closes #4076 Only that StateGraph model and two uses of "in dB" were changed. In addition to these changes the only other cases were in Modelica.Units.Conversions - cases such as the following:

function to_degC "Convert from kelvin to degree Celsius"
  extends Modelica.Units.Icons.Conversion;
  input SI.Temperature Kelvin "Value in kelvin";
  output Modelica.Units.NonSI.Temperature_degC Celsius "Value in degree Celsius";

I didn't update them, because:

• It seemed a bit tedious
• Without the unit the documentation would be kind of meaningless. "You send in a value and get out a value!"

It may be possible to reformulate it in a good way without the redundant unit but I don't know exactly how, and I don't view it as important.

[Harisankar-Allimangalath]

@beutlich can you please review the changes . Thankyou