Fixed bugs in ParmEst cov_est function and Estimator class

[slilonfe5]

Fixes # .

This PR fixes bugs in the Estimator class, cov_est, and _cov_at_theta functions of ParmEst

Summary/Motivation:

1. The number of data points should only be counted in the case where the measurement error is not supplied by the user
2. The check that ensures that the number of data points is greater than the number of unknown parameters should be carried out in _cov_at_theta and not in cov_est, as this is only needed when measurement error is not supplied by the user

Changes proposed in this PR:

• Ensure that the number of data points is only calculated for the SSE-with-unsupplied-measurement-error case
• Ensure that the check that compares the number of data points and the number of unknown parameters lives in _cov_at_theta

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[slilonfe5]

@blnicho @mrmundt @jsiirola @adowling2 This PR fixes two bugs to ensure that the number of data points and parameters are only evaluated for the SSE-with-no-supplied-measurement-error case. Two tests are failing on installation, while one is failing on solvers.

Kindly review. Very short PR. Thank you.