Change "lambda" to "lambda_soi" for python compatibility reasons and naming convention

[pfannkuch3n]

What is the problem?

In DHC-pipes models AixLib.Fluid.DistrictHeatingCooling.Pipes.PlugFlowPipeEmbedded and DHCPipe
Heat conductivity of material/soil is named "lambda" while other soil-related parameters ending with _soi. When writing modelica code with python "lambda" requires a workaround, since its a reserved keyword in python.

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

I would suggest change lambda to lambda_soi (would be, in my opinion, more consistent with remaining variables)