v1.3

Fix bug in building Z matrix. Add new APIs api_clear_meters, api_get_system_angular_stable_flag. Update implementation of bus frequency calculation. Reconstructure folder architecture and modify some benchmarks.
Update stepspy to add APIs is_system_angular_stable, get_(lines/transformers/hvdcs)between_lines(), set(generator/wt_generator/pv_unit/energy_storage/load/hvdc)_power(). Helps to stepspy module are added.
This version is compatible with -std=c++14 flag and can be complied with gcc>=4.9.2.
The attached libSTEPS.dll is for 64-bit Python 3 on Windows platform.