Remove extra spaces
Prepare Exercise 3

I have added the helper function note_duration() from the book text
and an assertion for the expected duration of the music provided
as example in the exercise.
Add actual value in assertion message
Complete exercise 3

I extracted the number of beats per measure and the reciprocal unity value
from the time signature string, then computed the duration of a beat and
multiplied by the total number of beats in the music.

I then converted the result (previously in seconds) to a duration in minutes.