Chemical inventory

This module is customized for managing an inventory of chemicals for a small laboratory. It defines a new node type "chemical", and a page that presents a listing of the chemical inventory.

Each chemical also gets a sequential, unique ID number for recording usage in a lab notebook. Chemicals that are used up can be edited and marked as "empty" so that they no longer appear except in the list of "empty" chemicals. This is intended for later reference back from lab notebooks to a specific chemical used in an experiment where the lot number, supplier, etc. may be important information for comparison between experimental results.