New coil curvature metric is ready for use

[zhucaoxiang]

Thanks to @thomasgbkruger , FOCUS has a new metric for reducing coil curvature. If you want to have a test, please pull the develop branch.

Detailed documentation (including the equations, how-to-use and notes can be found here. An example of using this new metric together with others to optimize HSX coils can be found here.

Any issues or questions can be discussed here. After fully testing, I will merge everything into master.