SIG and SDSIG

Added some bug fixes.

SIG and SDSIG

The SIG and SDSIG models presented in this version are as they were when published.
They have undergone some code restructuring to make the models easier to follow the structure and order.

Original release after publication of the SIG in Solar Energy

This version is maintained as it may have been used before the code rewrite.
This is not recommended to be used anymore, see latest version.