Replace `hh` in function names by `bg`/`fg`/`vg`

[lars-reimann]

What problem do you want to solve?

Replace hh in function names by bg/fg/vg, as discussed in https://github.com/iza-institute-of-labor-economics/gettsim/pull/654#issuecomment-1719414210.

Until #601 is available, it makes no sense to also rename the _hh parameters of the functions, since bg_id and fg_id are not available yet.

Todo

• [X] Pick an appropriate title.
• [X] Put Closes #XXXX in the first PR comment to auto-close the relevant issue once the PR is accepted. This is not applicable if there is no corresponding issue.
• [x] Document PR in CHANGES.md.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (419769f) 90.95% compared to head (dbc139f) 91.01%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #662 +/- ## ========================================== + Coverage 90.95% 91.01% +0.06% ========================================== Files 48 48 Lines 3140 3140 ========================================== + Hits 2856 2858 +2 + Misses 284 282 -2 ```

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