Add sin, cos and tan trigonometric functions

[lemm-e]

This PR adds sin, cos and tan trigonometric functions from the math package.

[l-zeuch]

This currently does not account for complex numbers, but they're also valid in custom commands (do not ask how I know...). The sine of a complex number can be defined as

$$ \sin (a+bi) = \sin a \cdot \cosh b + i \cos a \cdot \sinh b $$

I'd suggest we

1. add complex numbers as a valid input to these function as well using the definition above, or
2. also add the hyperbolic functions.

Personally I'm leaning towards both; the hyperbolic functions may have some interesting properties as well and given that we're already here, why not.

[l-zeuch]

The complex cosine can be defined as

$$ \cos (a+bi) = \cos a \cdot \cosh b - i \sin a \cdot \sinh b $$

[l-zeuch]

Similarly,

$$ \tan (a + bi) = \frac{\sin (a+bi)}{\cos (a+bi)} = \frac{\sin a \cdot \cosh b + i \cos a \cdot \sinh b}{\cos a \cdot \cosh b - i \sin a \cdot \sinh b} $$