feat: Add complex numbers

[sigmasternchen]

First draft for complex number (#33), please take a look.

Changes from the discussion on the issue:

• I added a public reciprocal function since I needed it internally anyway.
• The power function only takes positive integer exponents, because I used De Moivre’s theorem. Accordingly weighted_product also only supports integer weights. If anyone knows a more generic algorithm, please let me know.

Documentation is also still missing, and I didn't do any changes to the main maths module yet.

Please let me know what you think.

I apologize in advance for not being very responsive - I'm currently on vacation and probably won't have a lot of time to work on this.

- Sigma