Single operator constrain with `single_parameter_fit`

[giacomomagni]

This PR is to allow single operator constrain in single parameter fits. #64

Before constrain where not allowed in the multiple single_parameter_fits.

[codecov[bot]]

Codecov Report

Attention: 24 lines in your changes are missing coverage. Please review.

Comparison is base (06d1e3c) 42.29% compared to head (ce5314d) 41.88%.

Additional details and impacted files

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[giacomomagni]

@ecelada I think this is the proper fix to the issue you raised some weeks ago. I'm not sure if you can review, but if you can it would be great. Thanks

[ecelada]

Thanks! I will review but it's probably better if someone else does it as well