This package provides a set of commands \NewOdvVariant,
\NewPdvVariant etc. that can be used to define derivatives.
Each derivative comes with a great number of options that tweak
the derivative's format to your liking. The following types of
derivatives come readily defined: \odv Ordinary derivative,
\pdv Partial derivative, \fdv Functional derivative, \mdv
Material derivative, \adv Average rate of change, \jdv
Jacobian.
