Modeling documentation in sipnet.c (#134)

* Splits state structs into new state.h/c files, adds modeling documention to sipnet.c

* Doc tweak

* Doc updates

* Docs Braswell params

* Docs Braswell params redux

* Doc refinement

* Minor tweaks

* Updates, hopefully now complete for Braswell 2005

* Recovers sopilSIPNET pdf

* Updates for roots and microbes provenance

* Updates for root/microbe fluxes

* Updates - tweak microbe doc

* Updates for roots and microbes and some general clarity

* Updates for modeling provenance

* Adds addition citings

* More updates

* Apply suggestion from @dlebauer

* Updates for formatting

---------

Co-authored-by: David LeBauer <[email protected]>

Author: Alomir