changes to parallel tabs sections implementation:

- nParCir is now set using a parameter instead of using a record
- added some more divisions by nParCir
also cleaned up code a bit
I do not remember why do we use the variable RadSlaCha (which is the pipe spacing) for the convective resistance. Is this correct? especially for a correlation with Nu_D where I would expect the inside diameter being used. (Same for R_w_val_min)