Custom complexities of operators, variables, and constants

[MilesCranmer]

This change allows you to declare the complexity of operators, variables, and constants, to your liking. No more will all complexities be fixed as simply the number of nodes in a tree–now you can state that exp is more complex than + by passing the argument:

square(x) = x^2

options = Options(
    binary_operators=(+, *),
    unary_operators=(exp, square),
    complexity_of_operators=[exp => 5, (+) => 1, square => 1]  # Default is 1, if undefined
)

This will be factored into the search, and the most accurate expression at each complexity will be returned.

Todo's:

• Right now it seems there's a bug where the initial expression generated can be larger than your maxsize, and this gives a bounds error (since it's trying to store the expression in the hall of fame, but there's no corresponding bin).
• Document this.
• Enable this for PySR (optional)