Add example unit test

[gituser789]

No description provided.

[SevenOfNinePE]

Attributes are tested almost well. Missing is explicit the variable 'None'.
Data input are not tested well. There are several opportunity to test.
Below, you will find my ideas of a deep test with "mixed=integer+float":
"np_array ()" Pos: 6 Tests -> 1: (0 to 20e-3) (+- mixed) 2: (5e-3 to 20e-3) (+- float)
3: (-20e-3 to -5e-3) (positive mixed) 4: (-20e-3 to 0) (+- integer) 5: (-20e-3 to 20e-3) (+- mixed) 6: (-20e-3 to 20e-3) (+- mixed non equidistant)

"np_array ()" Neg: 3 Tests-> 1: (constant x-values) (+- mixed) 2: (Not ascent) (+- mixed)
3: (2 times same x-Value)
"mode" Pos: 4 Tests-> 'None' 'rad' 'deg' or 'time'
"mode" Neg: 1 Tests-> 'Wrong'
"f0" Pos: 3 Tests-> '1E5' 'None' '1E-12' (1E100 und 1E-100)?
"f0" Neg: 1 Tests-> '0'
"channel_label" Pos: 2 Test 'None' 'Anyword'
"channel_label" Neg: 1 Test 'float or integer'
"channel_unit" Pos: 2 Test 'None' 'Anyword'
"channel_unit" Neg: 1 Test 'float or integer'