Support eigvals for sparse hamiltonians

[soranjh]

Context:
Adds functionality for returning the eigenvalues of a sparse Hamiltonian. The eigendecomposition of sparse Hamiltonians is much more efficient with the scipy sparse linear algebra tools.

Description of the Change:
The qml.eigvals function is modified to recognize a SparseHamiltonian object and use scipy.sparse.linalg.eigsh for the eigendecomposition.

Benefits:
The new addition provides a very efficient way for obtaining the eigenvalues of a sparse Hamiltonian.

Possible Drawbacks:
scipy.sparse.linalg.eigsh computes k ( = 6 by default) eigenvalues and eigenvectors only.

Related GitHub Issues:
#2308

[github-actions]

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Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

Merging #2333 (a455d60) into master (d360afd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2333   +/-   ##
=======================================
  Coverage   99.39%   99.39%           
=======================================
  Files         229      229           
  Lines       17442    17448    +6     
=======================================
+ Hits        17336    17342    +6     
  Misses        106      106           

Impacted Files	Coverage Δ
pennylane/ops/functions/eigvals.py	100.00% <100.00%> (ø)

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[Jaybsoni]

Just a few small things, but other then that it looks good 👍🏼 Should be quite a good speed up!

[KetpuntoG]

Just a small comment on the doc, the rest I see it perfect! 💪

[ixfoduap]

Looks good! Just maybe good to add a short explanation on what the possible which methods refer to.