Support negative lags in RV calculation

[tsalo]

Detailed Description

Currently, only positive lags are supported in RV calculation:

phys2denoise/phys2denoise/metrics/chest_belt.py

Line 155 in cfced3b

delays = [abs(int(lag * samplerate)) for lag in lags]

However, apply_lags() supports negative lags just fine.

Context / Motivation

I require this for an analysis, and there is no real cost. In fact, as it is implemented, it could be confusing for users.

Possible Implementation

I think it's pretty straightforward to fix:

    delays = [int(lag * samplerate) for lag in lags] 

[smoia]

We can express the breaking change in the release description, if that's what worries you, but otherwise I would drop that abs() too!