Rename algorithm symbols

[pbrehmer]

Here we rename all algorithm symbols to have consistent capitalization according to their algorithm structs, see #368.

Note that I also renamed :iterative to :GKL since that is more consistent with how we name things for the iterative eigh solvers; there we call the solvers :Lanzcos and :BlockLanzcos.

[codecov]

Codecov Report

❌ Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...rithms/optimization/fixed_point_differentiation.jl	25.00%	3 Missing ⚠️
src/algorithms/ctmrg/c4v.jl	33.33%	2 Missing ⚠️

Files with missing lines	Coverage Δ
src/Defaults.jl	85.71% <ø> (ø)
src/algorithms/ctmrg/ctmrg.jl	88.15% <100.00%> (ø)
src/algorithms/ctmrg/projectors.jl	87.09% <100.00%> (ø)
src/algorithms/ctmrg/sequential.jl	98.41% <100.00%> (ø)
src/algorithms/ctmrg/simultaneous.jl	100.00% <100.00%> (ø)
src/algorithms/optimization/peps_optimization.jl	94.66% <ø> (ø)
src/algorithms/select_algorithm.jl	78.57% <100.00%> (ø)
src/algorithms/truncation/truncationschemes.jl	93.33% <ø> (ø)
src/utility/svd.jl	80.66% <100.00%> (ø)
src/algorithms/ctmrg/c4v.jl	82.07% <33.33%> (ø)
... and 1 more

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[lkdvos]

I think overall this looks great, I don't really have anything to add to this :)