Consider adding an assert for constructor in OpeningKey

[kevaundray]

This has been taken from Benedikt Wagner's document.

Link:

rust-eth-kzg/cryptography/kzg_multi_open/src/opening_key.rs

Line 36 in e043452

pub fn new(

Add assert that `coset_size` is not too large compared to the size of `g2s`;  since in `fk20/verifier.rs`, access `g2s` are accessed at `coset_size`.

Related to #246