Optimize waveform generation

[cjcolvar]

Changes propsed in this PR:

• FFmpeg downmix to mono wav instead of having waveform code check both channels for min/max peaks.
• Bring gather_peaks method into get_normalized_peaks so overall absolute value max can be computed while gathering peaks instead of needing a separate pass through the peaks after.
• Scale peaks in place instead of allocating another array
• Explicitly close both reader and IO instead of waiting for FFmpeg to terminate
• Avoid flatten since it is expensive when run so frequently

Note that the waveform fixture has slightly different values. I found that running the waveform job on a real item on mco-staging yielded different length and values even when run with the same code. But the small differences did not affect how it rendered in the SME so shouldn't be problem.

Resolves #5155

[masaball]

Looks good!