Added truthtable for ICs
Invalidlists defined during Initialisatn itself

For the 'truthtable()' method for ICs, it was necessary that the invalidlist be defines during initialisation of that IC.
Example for 'truthtable' method.
Merge pull request #167 from mohitgujarathi14/tt

Added Truthtable method for ICs
'Truthtable' method when applied can give the user a truthtable corresponding to the set of inputs and outputs pins specified by the user for a defined IC.