Adds LSQR and randomised solvers

[alexhroom]

This PR builds on top of #136 by adding two additional solvers:

• an LSQR iterative solver, based on a reverse communication implementation by Jennifer Scott
• a randomised sketch-and-precondition solver, primarily based on Blendenpik (Avron et al. 2010) but using the sketch-and-solve initial guess technique (outlined in Meier et al. 2023). Two sketching methods are available; either a simple random row selection, or a random row selection after using a discrete cosine transform to reduce matrix coherence (see Avron et al. above).