Error in find_rainflow_ranges for boundary cases

find_rainflow_ranges gives error for arbitrary numpy array. For example if the array doesn't contain any cycles - fatpack.find_rainflow_ranges(np.array([1,2,3,4,5]), return_means=True, k=2**14) produces error.


Reproduce the error with fatpack.find_rainflow_ranges(np.array([1,2,3,4,5]), return_means=True, k=2**14), then locate that function and inspect its handling of arrays containing no cycles. Done means the boundary case returns a valid result instead of raising an error, with coverage for the provided example.

Written by the indexing model from the issue text.