Enhancement features development

[jinningwang]

• Add function signature attr='v' to method Model.alter for altering parameter values without manual
  per unit conversion
• Add following methods to GroupBase for easier usage: get_all_idxes, alter, as_dict, and as_df
• Add two demo "add_RenGen.ipynb" and "replace_SynGen.ipynb" to show how to do renewable penetration via code.
  See folder andes/examples/demonstration
• Add a demo "manipulate_params.ipynb" to compare the differnce between set and alter
• Extend SynGen common parameters with bus, gen, Sn, Vn, and fn
• Extend StaticGen common parameters with bus

[jinningwang]

This PR, if successful, can address #579

[jinningwang]

@cuihantao This PR is ready to merge if it looks good to you

[cuihantao]

Jinning, it looks great! Let me take a closer look later today. I'll merge it very soon.

[cuihantao]

Thanks, Jinning!