[WIP] ParmEst bug fix: SSE_weighted should support heterogenous measurement errors

[adowling2]

Fixes # .

Summary/Motivation:

Small ParmEst bug (incorrectly) raises an AssertionError that measurements need to have the same length, which is not the case for SSE_weighted

Changes proposed in this PR:

• Allows heterogeneous measurement errors
• Adds a lightweight test to confirm that the AssertionError (or any other errors) are not raised

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Before removing the [WIP] / draft tag, I need to switch to "Reviewed/verified". As part of this, I want to discuss with @sscini and @slilonfe5 the names for the functions touched in this code.

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[adowling2]

Closing in favor of #3980