Added tikz commands for diagram components.
Added figure for the PSBC.
Corrected reference in the introduction.
Began describing PSBC.
Restructured commands files.
Moved figure files.
Corrected command includes in main file.
Finished PSBC_system graphic; small corrections.

* Finished the PSBC_system Tikz-graphic.
* Changed intensity [0,1] to valence [-1,1] in the description of the
PSBC.
* Specified hormone storage.