Integrate pypower solver

[dchassin]

New capabilities

Add support for fully integrated pypower solver.

• [x] Add bus class
• [x] Add branch class
• [x] Add gen class
• [x] Add load class
• [x] Add powerplant class
• [x] Add powerline class
• [x] Add gencost class
• [x] Add module globals
  • [x] version: specify the PyPower solver version
  • [x] solver_method: specify PyPower solver algorithm
  • [x] maximum_timestep: specify the maximum gridlabd timestep to allow between solutions
  • [x] solve_update_resolution: specify the minimum value magnitude to consider a change needing iteration
  • [x] baseMVA: specify the base MVA to use in per-unit conversions
  • [x] enable_opf: enables the OPF solver
  • [x] stop_on_failure: stop gridlabd if solver fails
  • [x] save_case: enable saving of solver input and output data
  • [x] controllers: specify the controller function python file
  • [x] controllers_path: specify the path to find the controllers file
• [x] Add support for OPF solver

Validation

• [x] Add IEEE test cases 14, 30, 39, 57, and 118.

Documentation

• [x] Add documentation for pypower module and py2glm converter.

Fixes needed

• [x] Fix powerflow::substation to support pypower::load object
• [x] Fix source/gridlabd.h to support global variable tests
• [x] Fix complex number converter to support python format
• [x] Fix gridlabd.in to support using gridlabd.rc files found in local and home directories.

See docs/Module/Pypower.md and docs/Converters/Import/PyPower_cases.md for documentation.