Add more metrics, esp. for Delay: Average Lagging

[obo]

Average Lagging is one of the popular methods of estimating delay; it is used, i.a. in SimulEval (https://aclanthology.org/2020.emnlp-demos.19/)

Add AL to SLTev in two variants:

• the original one (https://aclanthology.org/P19-1289/)
• the improvement which avoids negative values; see Section 5.4 of https://aclanthology.org/2022.iwslt-1.24.pdf