Add `.cis()` methods for Float and Complex<Float>

[PaulXiCao]

Closes #136 .

This adds the .cis() method for Float and Complex<Float>.

Adding it as a trait-based method allows for overloading compared to a free standing implementation.

The Float version was already available as a free standing function. It could be discussed if it should be deprecated/removed infavour of the method version. (Maybe part of a separate issue/pr?)