generalised unpack function after a model is solved

[MridulS]

fixes #457

Please ensure your pull request adheres to the following guidelines:

• [N/A ] Tests for new functionality/models or Tests to reproduce the bug-fix in code.
• Updated documentation of features that add new functionality.
• Update CHANGELOG.md with major/minor changes.

[sbenthall]

Is the idea that this is already covered by existing tests?

[MridulS]

nope, this is supposed to fail for the time being. I want to discuss this in the call today.

Currently we use unpackcFunc and this PR introduces unpackFunc (notice the missing 'c') so we would need to update the code examples where we use model.unpackcFunc() with something like model.unpackFunc('cFunc') to have a generalised unpacking function. model.unpackFunc('cFunc') does look clunky so maybe we can have something like model.unpack('cFunc')

[llorracc]

Don't merge until the functionality of the old unpackcFunc is restored, deprecation notification is added

[llorracc]

@MridulS

Thanks for fixing then merging this.