Complex

[S-Erik]

This pull request involves the following:

• Renaming of tests to better fit the naming in lib.rs
• Simplification on the implementation of the Imag trait for primitive types via the num_traits Num trait instead of using macros
• Bugfix in the arg function which used $\text{atan2}\left(\frac{y}{x}\right)$ instead of $\text{atan2}\left(\frac{x}{y}\right)$ where the complex number $z=x+iy$.
• Implementation of the From trait for complex numbers for primitive types via the Num trait.
• Typo in the Cargo.toml which caused that complex was not a feature

[JErnestoMtz]

Thank you a lot @S-Erik! that some serious improvement! And great job spotting the arg mistake