Add options to fix clipping and apply peak normalization to the generated soundscape

[justinsalamon]

Right now, it's very easy for the user to generate a soundscape the clips if they're not careful with their choice of ref_db and the SNR values they give to foreground events.

This PR adds two new optional parameters to both generate and generate_from_jams:

• fix_clipping - if True (default=False), will apply peak normalization in the case that the soundscape is clipping
• peak_normalization - allows the user to apply peak normalization (abs(max) sample = 1) even if the soundscape isn't clipping

This PR addresses #131

Checklist:

generate:

• modify soundscape audio
• modify event audio
• jams: update ref_db
• jams: store fix_clipping and peak_normalization bools
• jams: store scale_factor?

generate_from_jams:

• modify soundscape audio
• modify event audio
• jams: update ref_db
• jams: store fix_clipping and peak_normalization bools
• jams: store scale_factor?

testing:

• peak_normalize
• generate
• generate_from_jams

[justinsalamon]

NOTE: missing unit tests, coverage at 98% right now.

[pseeth]

Looking good so far, I've left some comments and thoughts.

[pseeth]

If we can unpin pyrsistent in this PR as well, that'd be great. We pinned it to be compat with 2.7 and 3.4, but we dropped support for those a few versions ago, so we can unpin.

[pseeth]

Looks pretty good, there are a few tiny things that should be looked at before we merge, I think. See comments.

[justinsalamon]

ok @pseeth I believe I've addressed all of your comments. Let's CR and SqM.