Displayin the mean sound speed in the database tab

[eriffon]

We have a requirement to quickly access the mean sound speed (weighted harmonic mean) of a cast without having to explicitly call the profile statistics calculation in SSM. A new column in the table of the Database tab would fulfill this requirement. This PR is a prototype solution. However, the weighted harmonic mean calculation is refreshed at every table update, which is far from optimal. A better solution would be to store the mean sound speed when the cast gets read into SSM and is permanently stored in the profile.Metadata class (or some other member of the Profile class). That would probably require a change to the SQlite3 data structure, wouldn't it? I presume that this would break backwards compatibility? I just wanted to hear some thoughts on this. Thanks.

[giumas]

@eriffon, I would avoid to store in the database a value that can be recalculated from the samples. The main risk is that the stored value gets outdated in one of the many ways that the profile can be edited.

An alternative to explore would be to have the weighted harmonic mean also as a "cached property": https://docs.python.org/3/library/functools.html#functools.cached_property
This way the database tab may use such a value knowing the risk of returning an outdated value. Maybe we can also try to add a mechanism that always invalidate cached properties when a profile is edited.

[eriffon]

@giumas, thanks for your thoughts and your suggestion to use a "cached property". I will look into this and modify the PR accordingly to get a prototype working.