Added a least squares solver in pd_solve()

[zyyk78]

Description

Added a least squares solver to the pd_solve function to prevent errors when np.linalg.solve(a, b) fails, without having to directly modify the ridge regression regularization coefficient.

Fixes #1524

Type of change

• Bug fix (non-breaking change which fixes an issue)

Branching

• All PRs should be made against the dev branch. The main branch is not often merged back to dev.
• If you want to get your PR out to the world faster (urgent bugfix), poke pgunn to cut a release; this will get it onto github and into conda faster

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caimanmanager test

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caimanmanager demotest

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