Add 'assign' method to ease symbol substitution

[Pierre Denis]

Given a Lea instance using SymPy symbols,

x = lea.binom(2,'p') + lea.binom(2,'q')

the symbols can be replaced by actual values using the Lea.subs method, which works as sympy.Expr.subs method:

x.subs({'p': 0.8, 'q': 0.6})

It is proposed to add a new assign method, so this can be done easier using keyword arguments:

x.assign(p=0.8, q=0.6)

[Pierre Denis]

Add Lea.assign method, extend Lea.subs method, make misc minor fixes (refs #81)

→ <<cset 90cab01d5ab4>>

[Pierre Denis]

Done in Lea 4