Album flexible attributes modifications are not inherited to tracks

[JOJ0]

As discussed in #4742, changing flexible attributes via a beet modify -a command does not inherit the changes to the tracks of the album.

Changing a fixed attribute on an album does inherit the changes to the album-tracks though.

Problem

Running

$ beet mod -a flex_attr=new_value search term

does not change flex_attr on the album`s tracks,

while running

$ beet mod -a genre=new_value search term  # genre is a fixed attribute available to albums and album-tracks

does change genre on the album's tracks.

Proposed solution

• Streamline the behaviour of beet modify -a - Fixed attributes changes as well as flexible attributes changes should be inherited to album-tracks.
• Add a CLI option that prevents inheritance, allowing changing fixed/flex attributes on an album only

[JOJ0]

@sampsyo Another use-case I'm brainstorming about: There are situations where an album attribute has a value already, and trying to fix its items is not possible when searching on album level. Eg:

beet ls -f '$genre' -a my album

tells me a value of good genre

while

beet ls -f '$genre' my album tracks

tells me values of messed up genre for some or all of the album's tracks

Now trying to fix genres ob album-tracks issuing

beet mod -a genre="good genre" my album

will tell me that the album's level is fine already an no modificiations are required.

I could add a --force option to modify in the PR I opened, what do you think?