Unify `LaplaceDiffusion1D` and `LaplaceDiffusion2D` BCs

[jlchan]

Closes #1418

[codecov[bot]]

Codecov Report

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

Project coverage is 89.55%. Comparing base (b4fb1db) to head (553bcca).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1421      +/-   ##
==========================================
+ Coverage   89.52%   89.55%   +0.03%     
==========================================
  Files         448      448              
  Lines       35995    35979      -16     
==========================================
- Hits        32223    32219       -4     
+ Misses       3772     3760      -12     

Flag	Coverage Δ
unittests	89.55% <ø> (+0.03%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[ranocha]

@sloede What's your take on this? If I remember correctly, we wanted to keep some dimension-agnostic code like this in each dimension-specific source file to make it easier to read, e.g.,

https://github.com/trixi-framework/Trixi.jl/blob/f404661aff18c8b39f339918218ebfcd118e39d6/src/solvers/dgsem_tree/dg_1d.jl#L479-L483

I would like to see a review from @sloede before merging this.