Allow creation of a KoopmanRegressor object from a Koopman matrix

[sdahdah]

Desired Behavior

Given a Koopman matrix U, create a KoopmanRegressor object that functions as if it was fit with pykoop.

Proposed Solution

Along these lines:

class DataRegressor(KoopmanRegressor):

    def __init__(self, U):
        self.U = U
        
    def fit(X, y):
        self.n_features_in_ = ...
        self.coef_ = U.copy()