add handling of $\alpha\beta$-synergy

[MathisWetterwald]

add $\alpha\beta$-synergy.
To test it out :

import dit
from syndisc.pid import PID_SD_beta

def my_system():
    pmf = [1/8]*8
    outcomes = ['0000','0011', '1100', '1011', '0101','0110','1101','1010']
    d = dit.Distribution(outcomes, pmf)
    return d

print(PID_SD_beta(my_system, 2, method='polytope', table='1D'))

The '2' means $X$ is the two first bytes, $Y$ is the $2$ last. If was $3$, $X$ would be the $3$ first bytes, $Y$ the last one.
method : 'polytope' or 'scypy'. if scypy, uses functions in scypy to optimize. If 'polytope', computes the polytopes. 'polytope' is faster.
table : '1D' or '2D'. The way to display data. If '1D', uses the print function in dit. If '2D', builds a 2-dimensions table.