This PR

• resolves Definition of rtol in matrix_rank, lstsq and pinv #216 by updating the guidance for rtol to accommodate non-SVD-like algorithms. Namely, instead of a strict cutoff of less than or equal to, this PR updates the guidance to say "approximately less than or equal to" to allow conforming implementations some wiggle room should they choose a non-SVD-like algorithm for implementing pinv. For matrix_rank, the API is already defined in terms of singular values, so, in practice, an implementation is likely to be SVD-based; in which case, the more strict guidance is likely to hold.
• in general, the previous guidance should hold true, given the prevalent usage of LAPACK; however, array libraries should be able to use alternative algorithms in accordance with the design principle to be "implementation agnostic". A future refinement could be to define the tolerance in terms of condition numbers, but that is left to future consideration.