Improvements in `OpSum` to `TTN` conversion

Followup to #116:

• Replace MatElem and QNArrElem with FillArrays.OneElement.
• Rename determine_val_type to coefficient_type.
• Default OpSum coefficient type to Float64, require users to specify OpSum{ComplexF64} if they want that.
• Check / Improve compatibility with feature set of OpSum to MPO conversion in ITensors: support multi-site operators, ensure sorting comparisons work and are implemented consistently with ITensors implementation, implement all relevant sorting w.r.t to traversal order of tree instead of site-labels to ensure compatibility with arbitrary vertextype.
• Copy ITensors functions being used in ttn_svd like ITensors.determineValType, ITensors.posInLink!, ITensors.MatElem, etc. to ITensorNetworks.jl and update their style. Functions like ITensors.which_op, ITensors.params, ITensors.site, ITensors.argument, etc. that come from the Ops module related to OpSum shouldn't be copied over.
• Split off logic for building symbolic representation of TTNO into a separate function.
• Move calc_qn outside of ttn_svd.
• Use sparse matrix/array data structures or metagraphs for symbolic representation of TTNO (for example NDTensors.SparseArrayDOKs may be useful for that).
• Split off logic of grouping terms by QNs.
• Factor out logic for building link indices, make use of IndsNetwork.
• Refactor code logic to first work without merged blocks/QNs and then optionally merge and compress as needed.
• Support other compression schemes, like rank-revealing sparse QR.
• Implement sequential compression as opposed to the current method which uses parallel compression (i.e. right now it compresses each link index effectively independently) to improve performance.
• Allow compression to take into account operator information (perhaps by preprocessing by expanding in an orthonormal operator basis), not just coefficients.
• Handle starting and ending blocks in a more elegant way, for example as part of a sparse matrix.
• Handle vertices without any site indices (internal vertices, such as for hierarchical TTN).
• Make sure the fermion signs of the tensors being constructed are correct and work with with automatic fermion sign system.

Start with ttn_svd and the OpSum-to-TTN conversion described as a follow-up to #116, then compare the OpSum-to-MPO conversion in ITensors. The checklist spans type changes, symbolic TTNO construction, QN grouping, compression, traversal ordering, and fermion signs, so the relevant entry points and tests must be identified before work begins. Done is not defined as a single acceptance criterion.