Lanczos-based `spectrum` method

[matteosecli]

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Description

Lanczos-based spectrum method.

Related issues or PRs

Resolves #463.

Additional context

This PR implements the non-symmetric variant of the Hermitian Lanczos method described in Koch2011.

The pseudo-code for the non-symmetric Lanczos algorithm can be found in Algorithm 6.6 of Saad2011, with the difference that this implementation works directly with row/col vectors (i.e. bras & kets) instead of col vectors only.

At each step, the running estimate for the result is updated via a Wallis-Euler recursion to avoid recomputing the whole continued fraction each time, and the convergence is evaluated accordingly.