Expand possible solutions.

For the four variables R1, R2, Vi and Vo;
Expand functionality to solve when any two possible inputs values are known.
I.E.    For 4 input values, be confused.
        For 3 input values, solve for 1 value.
        For 2 input values, solve for 2 values.
        For 1 input values, be confused.
Update documentation.
Various scattered uncoordinated changes ..