Add clef map

[leleogere]

This PR adds a new clef_map function, in the continuity of time_signature_map and key_signature_map. As at each time point there are as many clefs as staves, for a n-staves part, the map function will return an array of shape (n, 4):

np.array([
    [1, 0, 2, 0],  # treble clef on staff 1
    [2, 1, 4, 0],  # bass clef on staff 2
    ...
    [n, 2, 3, 0],  # ut3 clef on staff n
])
# each line is: [staff_no, clef_id, line, octave_shift]

[sildater]

Very nice contribution! I changed base to a new testing branch, so I can easily commit minor changes and test against a file with missing clef. I'll open a new PR as soon as I have done this.