Change statcast pool from process to thread; fix TypeError

[TheCleric]

Fixes #229 by converting out ProcessPoolExecutor to a ThreadPoolExecutor. ProcessPoolExecutor is a little more picky about where it runs.

As well I was getting a TypeError in statcast_pitcher_spin.py when running the tests (python 3.7.9):

pybaseball/statcast_pitcher_spin.py:135: in find_phi
    df['phi'] = df['phi'].round(0).astype('int64')
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = 0       190.754943
1       300.794706
2        243.13629
3        288.17292
4       291.559998
           ...    
4215...5478
4216    448.531754
4217    436.495487
4218    440.636358
4219    434.056495
Name: phi, Length: 4220, dtype: object
decimals = 0, args = (), kwargs = {}

    def round(self, decimals=0, *args, **kwargs) -> Series:
        """
        Round each value in a Series to the given number of decimals.
    
        Parameters
        ----------
        decimals : int, default 0
            Number of decimal places to round to. If decimals is negative,
            it specifies the number of positions to the left of the decimal point.
        *args, **kwargs
            Additional arguments and keywords have no effect but might be
            accepted for compatibility with NumPy.
    
        Returns
        -------
        Series
            Rounded values of the Series.
    
        See Also
        --------
        numpy.around : Round values of an np.array.
        DataFrame.round : Round values of a DataFrame.
    
        Examples
        --------
        >>> s = pd.Series([0.1, 1.3, 2.7])
        >>> s.round()
        0    0.0
        1    1.0
        2    3.0
        dtype: float64
        """
        nv.validate_round(args, kwargs)
>       result = self._values.round(decimals)
E       TypeError: loop of ufunc does not support argument 0 of type float which has no callable rint method

So I followed the recommendation to make sure the column is cast to a float first before rounding.