Use BFGS as default in optimization for function space operators

[JoshuaLampert]

As discussed in #415 (I also noticed this already some time ago, but never updated the default here), BFGS usually performs better than LBFGS for us at least for the smaller to medium size problems. In the tests it usually converges faster than LBFGS both in terms of iteration count and function evaluations, it runs also faster, and usually often even converges to a slightly smaller minimum. The latter even allows us to use slightly tighter tolerances in the tests.
I did not change the default linesearch algorithm because for BFGS the difference between them is not as strong as for LBFGS and it would require us to put LineSearches.jl into the extension (we could do that because it would automatically be loaded since Optim.jl loads it, but IMO it would not be so nice).
Since this feature is marked as experimental

SummationByPartsOperators.jl/src/function_space_operators.jl

Lines 61 to 62 in ede5990

	!!! warning "Experimental implementation"
	This is an experimental feature and may change in future releases.

I think it is fine to change the default.

Closes #415.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.09%. Comparing base (ede5990) to head (a3aa3ef).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #416   +/-   ##
=======================================
  Coverage   93.09%   93.09%           
=======================================
  Files          42       42           
  Lines        6807     6807           
=======================================
  Hits         6337     6337           
  Misses        470      470           

Flag	Coverage Δ
unittests	93.09% <ø> (ø)

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[coveralls]

coverage: 93.088%. remained the same — JoshuaLampert:default-opt-alg-fsbp into ranocha:main

[ranocha]

Thanks