def remove_low_std(X,std_val=0.01):
    """
    Function that marks columns in the feature matrix based on their standard deviation.

    Parameters
    ----------
    X : pandas DF
        feature matrix
    std_val : float
        minimal standard deviation for one feature to not be marked
        
    Returns
    -------
    list
        feature names that do not meet the standard deviation requirement
    """
    rem_f = []
    
    # Get the standard deviation per column
    std_dist = X.std(axis=0)
    
    # If below filter; add to list
    rem_f.extend(list(std_dist.index[std_dist<std_val]))
    return(rem_f)