All the windows in CQT

[bmcfee]

This PR implements #399:

• switched -3dB cutoff to equivalent noise bandwidth
• added window option to all cqt functions
• modified __float_window to call get_window instead
• removed default option from window_bandwidth; bad windows should fail noisily.

TODO

• tests

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This change is

[bmcfee]

Note: this PR also introduces a minor backwards incompatibility because it now forces asymmetric windows (through get_window introduced in #413), where the previous version would use symmetric windows everywhere.

I don't expect this to matter in practice, but we should note it in the changelog.

[bmcfee]

I think this one's ready for CR -- @dpwe @lostanlen @ebattenberg @ejhumphrey anyone want to claim it?

[ejhumphrey]

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Reviewed 1 of 2 files at r1, 2 of 2 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved.

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