Rhythm: After setting the IOIs, attributes like ``n_bars`` return incorrect value

[Jellevanderwerff]

When doing the following:

r = thebeat.music.Rhythm([500, 500, 500, 500])
r.iois = [500, 250, 500, 500]

attributes like n_bars return the incorrect value (because the attributes are saved in the constructor of the Rhythm class).

[YannickJadoul]

We probably want to turn this (and even others, like iois, etc.) into properties?

See also the is_played and other attributes in the Melody class/constructor; similar story there: #102 (comment)