add friction model "Hofer"

[jkisse]

The friction factor "Hofer" is introduced. It is an explicit implementation of Colebrook, used by some established Gas Grid Simulation tools.
Please refer to the added paragraph in the docs for more details.

[dlohmeier]

Looks interesting. I just thought that the transition area is relevant for different models. Maybe we should think about how to correct equations for laminar / turbulent flow. I think there is an issue on this topic and this implementation could be a nice approach.

[jkisse]

Thanks @dlohmeier!
I'd also appreciate your opinion on the usage of np.divide etc. in numba functions, e.g. here. It seems to me as if the advantage of array-processing cannot be used in the for-loops, thus it might be slightly more efficient to use the native "/" directly instead of np.divide? Did you investigate that a bit when you implemented the numba-functions?

(analogously for np.abs and np.power)

[dlohmeier]

As far as I can recall, numba was good at exactly this: identify numpy code within jit-decorated functions and compile them correctly. I always found it more readable and wrt. timings I remember that I found no difference or a slight advantage of the numpy ufuncs over native code. It might have changed with newer numba versions.