Fix 'fps' not defined for 'CompositeAudioClip' at initialization

[mondeja]

Closes #1457

• Refactor initialization of CompositeAudioClip to make it more readable, fix minor issues and use generators building attributes.
• Refactor concatenate_audioclips in corcondance.
• Fix error of fps not defined in CompositeAudioClip initialization if created directly or using CompositeVideoClip.
• Fix some AudioClip tests.

The fps of the CompositeAudioClip will be the maximum fps attribute of their clips, if any has the attribute setted. If an user uses different fps for their clips, no error neither warning is raised so is their responsibility to handle this inconsistency, because could be the desired behavior in some experimental compositions.

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it
• I have formatted my code using black -t py36

[coveralls]

Coverage increased (+0.02%) to 65.906% when pulling 7a63f1e on mondeja:rewrite-CompositeAudioClip into 7183eaa on Zulko:master.