Check the values of z/L

[AlexDo1]

L: Monin-Obukhov length
z: measurement height

The results of z/L are differing from the results in the TK2 data (assumption: z = 2).
The results given by the function obukhov_length_calc() could be wrong but the value of z in the TK2 data is also unknown.

Originally posted by @mmaelicke in #15 (comment)

[mmaelicke]

We should resolve this in the mid-term by a unitttest.

Each of the meteo-functions will be tested in a test, independent of the eddy data. Then, from my point of view, our functions are fine.