Amplpower

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* Return results for any status

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* Add formulations of OPF to docs

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* Tightest computation of bigMs for AC OTS.
* Tightest bounds for variables cosft and sinft.
* Add violations of AC constraints to results.
* Test use open-source solvers.

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* Corrected voltage results for AC jabr relaxation (acjabr).

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* Included default COST2 for generators (gencos) if not provided.
* Corrected voltage results for AC rectangular (acrect).

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* Added support for solving optimal power flow (OPF) problems: DC OPF, AC OPF (both rectangular and polar coordinates) and AC relaxation proposed by Jabr.
* Added functionality for solving the optimal transmission switching (OTS) problem: Big-M and non-linear formulations. Option to include or exclude connectivity constraints for the OTS problem