Fix LocalOperator constructor for indices of type Tuple{Int,Int}

[pbrehmer]

Previously, tuple-typed indices would lead to an error since the inner constructor uses a PeriodicArray (which apparently can only be indexed using CartesianIndices). Now, I convert tuple indices to CartesianIndices such that internally always the same type is used but users can still use tuple indices (which we also do in the docstring).

[lkdvos]

This is not something specific to PeriodicArray actually: if you use a tuple of Int you need to splat them to make the indexing work. What was actually breaking before?

[pbrehmer]

The thing that made it break was this line:

for i in 1:length(inds)
    @assert space(operator, i) == plattice[inds[i]]
end

since the tuples weren't splatted.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/operators/localoperator.jl	80.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/algorithms/select_algorithm.jl	95.83% <100.00%> (ø)
src/operators/localoperator.jl	59.09% <80.00%> (+0.60%)	⬆️

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