Rework Measure Repeats

[drumnkyle]

• Changed diagram for repeats
• Implemented changes
• Started implementing tests

Here are the changes:

• Introduced ImmutableMeasure protocol
• Measure now implements ImmutableMeasure and adds mutating methods
• RepeatedMeasure implements ImmutableMeasure and adds nothing
• MeasureRepeat now has an expand() method that generates an array of NotesHolder containing both Measures and RepeatedMeasures.

[drumnkyle]

• [x] Implement Changes
• [x] Make structs conform to Equatable protocol
• [x] Write Tests

[drumnkyle]

@migue48: Please review this code before I push it to see if it all makes sense as per our discussion.

[drumnkyle]

Produced issue #8 from this change.