Add docstrings for the balancer_math.py

[danlessa]

It would be useful to have docstrings for each function explicitly telling what each operation means. This would make it easier to people use the model as a source of truth and as a educational material.

It could be something like that:

def calc_out_given_in(*args):
 """
 Calculate the amount of output tokens retrieved when selling a input amount of tokens

 Arguments
 token_amount_in: amount of input tokens to be sold 
 (...)
 
 Output
 token_amount_out: resulting output tokens from trading
 token_amount_in_fee: fee to be paid on input tokens
 
 Math
 $$
 (latex here)
 $$
 
 References
 - https://balancer.finance/whitepaper/#out-given-in

 Examples
 >>> calc_out_given_in(...)
 BalancerMathResult(...)