Extend .subs to accept keyword type syntax, i.e. .subs(a = 2*c, b=1, d=sin(x),...)

[gutow]

For the most naive users the following behavior would be the most natural for substituting into an equation. This behavior is implemented in the SageMath package, but I believe it uses preparsing.

The basic idea is that the following two statements should be equivalent given an equation eq1 = Eqn(a,b/c):

• eq1.subs({a:1,b:c})
• eq1.subs(a=1,b=c)
  It may be possible to play tricks with **kwargs to make this work.