Add thermal calculation results to pipe

[dlohmeier]

I think it would be very useful to add a clear result to the pipe result table about the heat balance. It can be used for checking the correctness of the calculation (i.e. mdot * c_p * (T_1 - T_0) = Q_ext - alpha*(T_ext - T_m), when assuming T_1/T_0 as start / end temperatures at the pipeline, Q_ext as external heat source and T_ext - T_m as temperature difference between inside and outside) and for displaying this result in graphics, which would be really a nice feature. It can be calculated, but I think that this is not always obvious.