Add logger warning for non-PSD covariance matrix

[slilonfe5]

Fixes # .

Added a logging warning for the case when the computed covariance matrix is not positive semi-definite (PSD).

Summary/Motivation:

When a model is not structurally identifiable, the computed covariance matrix may not be PSD. This PR adds a warning message to the logging output when such a case is encountered.

Changes proposed in this PR:

• Add an if statement that checks if the covariance matrix is PSD
• Add a logging warning if the covariance matrix is not PSD

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