Add Inorganic fertilizer - chemical area densities

[KrishnaTO]

As chemicals linked with CHEBI's role-based term fertilizer*, the task is to create and link each chemical's area densities (as entities) with chemicals previously imported to AGRO's inorganic fertilizer.

• Ex. magnesium oxide has subclass 'has role' some fertilizer

[KrishnaTO]

Request CHEBI to add synonym 'inorganic fertilizer role'

[KrishnaTO]

...amount changed to amount applied
synonym added
...amount applied = ...area density = ...

decided further to keep area density, despite #24. Although do accept future revisions required for exact/concrete measurement principles for Ontologies.

[KrishnaTO]

Added in PR #123