allow periodic FDSBP operators

[ranocha]

I enabled the use of fully periodic FDSBP operators. It would be nice if you could take a look, @gregorgassner, since both @andrewwinters5000 and @sloede are on vacation.

[sloede]

LGTM overall with only minor suggestions/questions

[codecov]

Codecov Report

Merging #1570 (2bfcb1a) into main (07981c3) will increase coverage by 7.00%.
The diff coverage is 79.55%.

@@            Coverage Diff             @@
##             main    #1570      +/-   ##
==========================================
+ Coverage   89.15%   96.15%   +7.00%     
==========================================
  Files         383      384       +1     
  Lines       31949    31998      +49     
==========================================
+ Hits        28483    30767    +2284     
+ Misses       3466     1231    -2235     

Flag	Coverage Δ
unittests	96.15% <79.55%> (+7.00%)	⬆️

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

Impacted Files	Coverage Δ
examples/tree_1d_fdsbp/elixir_advection_upwind.jl	100.00% <ø> (ø)
src/solvers/fdsbp_tree/fdsbp.jl	100.00% <ø> (ø)
src/solvers/fdsbp_tree/fdsbp_1d.jl	90.14% <78.57%> (-1.40%)	⬇️
src/solvers/fdsbp_tree/fdsbp_2d.jl	90.96% <78.57%> (-1.16%)	⬇️
src/solvers/fdsbp_tree/fdsbp_3d.jl	93.07% <78.57%> (-1.14%)	⬇️
.../tree_1d_fdsbp/elixir_advection_upwind_periodic.jl	100.00% <100.00%> (ø)

... and 36 files with indirect coverage changes

[sloede]

LGTM, thanks!