Uniform output of ecg_quality algorithms

[LucaCerina]

Describe the bug
Currently the Zhao2018 algorithm in ecg_quality() function returns a string of the quality class, while others return a number. This behaviour complicates pipelines where the quality algorithm may be interchangeable.

To Reproduce
Behaviour always happening since it's a design choice.

Expected behaviour
Best case scenario is that all ecg_quality() returned a metric factor (e.g. a score, that can be also discretized, from 0 to 100). Otherwise it's better if it returns a number by default (with proper documentation on the meaning) and only optionally the string.

Screenshots
None applicable

System Specifications
Independent of system specifications.