Add BIGASC plug-in

[bobu]

I retired with several algorithms that had been written, but which remained in a primitive state. (For example, BIGASC, which identifies the limiting elements and exchanges in a collection of multi-element networks (e.g., C, N, & P) describing an ecosystem. Ursula said she was going to try to enlist a mathematician/programmer to rehabilitate these programs into some contemporary language, like R. The thought came to me today that they might make useful "plug-ins" for ECOPATH?