Implement pause length metrics

[clarkenj]

Linked to #2.

Potential features:

• Average pause length
• Ratio of short to long pauses?
• Percentage of pauses which are short or long?

References (PPA):

• Baqué & Machuca (2023): dense results, but found differences in pause duration between PPA substypes and controls.
• Ossewaarde et al. (2024): nfvPPA speakers use more short pauses relative to long pauses than control speakers, and the duration of short and long pauses is longer; svPPA speakers use more longer pauses relative to short pauses.

May be less important in AD:

• Lofgren & Hinzen (2022): In both mild and moderate AD, pause frequency was different from controls but not duration (manual coding). Also looked at syntactic position of pauses.