Have a short convenience method to get probability of truth

By default, Lea does not display individual probabilities but probability distributions; this is so even for boolean variables (i.e. probability of False and probability of True). For boolean variables, it's very common and natural to require only the probability of True, as a fraction or float. Lea provides general methods to do that: p (fraction) and pmf (float):

>>> rain = Lea.boolProb(1,4)
>>> rain.p(True)
1/4
>>> rain.pmf(True)
0.25
>>> die = Lea.interval(1,6)
>>> ((2 <= die) & (die < 6)).p(True)
2/3
>>> ((2 <= die) & (die < 6)).pmf(True)
0.6666666666666666

It would be nice to allow for a shorter, handy, syntax, that avoids the True argument. The following is suggested (note the capital P to avoid confusion with p method):

>>> rain.P
1/4
>>> rain.Pf
0.25
>>> ((2 <= die) & (die < 6)).P
2/3
>>> ((2 <= die) & (die < 6)).Pf
0.6666666666666666

Also, these methods could be provided as pure functions, enabling a syntax close to standard mathematical notations:

>>> P(rain)
1/4
>>> Pf(rain)
0.25
>>> P((2 <= die) & (die < 6))
2/3
>>> Pf((2 <= die) & (die < 6))
0.6666666666666666

Syntactic sugar matters!

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