Log Message:
math/kalker: update to 2.0.4

 - Prevent stack overflow crashes by limiting the recursion depth
   (thanks to @kvnxiao)
 - Fix comprehensions breaking if a previously evaluated comprehension
   encountered an error in a nested expression
 - Fix expressions like x^x not working inside functions
 - Allow a leading dot, eg. .25
 - Allow expressions like sin2x
 - Fix incorrect unit being returned from some trigonometry functions