v19.05 minor fixes (power-grid-lib#24)

updates for v19.01 (power-grid-lib#19)

PGLib-OPF v18.08 (power-grid-lib#14)

Update to v18.08

v18.06 Update

minor update
- closes power-grid-lib#4 
- closes power-grid-lib#6
- update to PowerModels v0.7

First Release of PGLib-OPF (power-grid-lib#1)

* adding initial benchmarks using PowerModels v0.4 and PowerModelsAnnex v0.0.5
* added repo metadata files