Fix: allowing renaming of filter windows

[sortega87]

A small fix that allows to rename filter windows. For instance:

    wave_type = "inertio_gravity"
    w_min, w_max = (0.3, 0.8)
    k_min, k_max = (-15, -1)
    h_min, h_max = (12, 50)         #Equivalent depths
    westward_inertio_gravity = get_wave_filter_window(wave_type,
                                                      k_min, k_max,
                                                      w_min, w_max,
                                                      h_min, h_max)       
    westward_inertio_gravity.name = "westward_inertio_gravity"

[sortega87]

The changes are also implemented in #5