Add an `apply_rolling_window` helper to pd_utils.

[bmmalone]

The idea is that this function applies a function to overlapping rows in a data frame (that is, a "rolling window"). A basic implementation is as follows:

def apply_rolling_window(df, func, window_size, progress_bar=False):
    
    it = range(len(df))
    
    if progress_bar:
        it = tqdm.trange(len(df))
        
    ret = [
        func(df.iloc[i: i+window_size])
            for i in it
    ]

    return ret