Module for various helper functions

[Trybnetic]

As pointed out in #5, there are some functions like _create_time_vector in the io module or

def calculate_cutpoints(vec):
    cutpoints = np.where(vec[:-1] - vec[1:] != 0)[0] + 1
    return cutpoints.reshape(cutpoints.shape[0]//2,2)

that are needed for different functions in paat.io or paat.wear_time, but are also useful for other purposes. We might want to organize these functions in a new module at some point. This issue is opened to keep that in mind