createDiode

Diode are now created based on Diode models
Cleaned all print statements
DCNonLinEquation
DCNonLinEquation update
NonLinCircuitElement

Created a basic class for Non-linear circuit elements
getNonLinContribution

now non circuit element may apply their contribution in their own
method.
Diode's getNonLinContribution has been updated.
Small Diode Mistake

made a small change in getNonLinContribution of Diode.java
getNonLinContribution

modified a bit the method