Add positive definiteness check of `theta_vcov` to `h_mmrm_tmb_check_conv()`

[danielinteractive]

Problem:
It is possible that other convergence checks pass, but theta_vcov is not positive definite. Downstream, this causes then the problem that the Satterthwaite d.f. can be negative, failing assertions.

To do:

• Add positive definiteness check of theta_vcov to h_mmrm_tmb_check_conv() in https://github.com/openpharma/mmrm/blob/main/R/tmb.R#L303
• Test it (by mocking such a case)
• Add a small NEWS entry (bug fixes)