Implement nonlinear formulas as residuals

[mikofski]

Related to #61 and #63
If a formula is flashed as nonlinear buy setting islinear = False then the formula should be written as a residual, ie: it should evaluate to zero

deg f_nonlinear(a, b, c):
    Vd = V - I*Rs
    Ish = V/Rsh
    Id = Isat * (exp(qVd/(nkT)) - 1)
    return I - Id - Ish

Then the default nonlinear calculator, either Levenburg-Marquardt out Newton-Raphson is used with all residuals

Also this means that formula needs an additional attribute called retval that indicates which argument to solve for

Or it will assume the first argument.