Heterostructure optical gain calculator

Updated version of gain calculator with some bugs fixed and new features are added

Updated version of program presented in http://www.mathworks.co.uk/matlabcentral/fileexchange/33353-optical-gain-calculator-for-all-iii-v-semiconductors-including-type-i-and-ii-heterojunctions
Some calculation bugs are fixed.
Parameters for III-nitrides wurtzite structures are added.
II-VI materials parameters are added.