consolidate.add_depth outputs to work with compute_MVBS

[leewujung]

In the overhaul of compute_MVBS (currently in the working in #1124) the variable water_depth is propagated down in the MVBS dataset. We should review what variables are needed to be propagated down, now that we have standardized the z-axis variables in v0.8.0, and some more improvements/refinement of add_depth will be introduced soon by @emiliom.

Relevant code section:
https://github.com/lsetiawan/echopype/blob/5a963bd397980583827bac2b6ba0b4bdd6ccf8cc/echopype/commongrid/api.py#L164-L166
This is in the PR code so may change in the future.

[emiliom]

Note: the variable that's now propagated is called water_level, not water_depth.

[leewujung]

This will be addressed potentially under #1205 or another PR that addresses the same thing.