Add a coefficient function

[dpsanders]

Add a coefficient function that extracts the multiple of a given subexpression in an expression,

e.g.

coefficient(3x + 2y, x) == 3

coefficient(x * y, x) == y

coefficient(x^2 + y, x^2) == 1

[dpsanders]

This should not use derivatives but should just extract the multiplier.

[YingboMa]

Duplicate #216