Added framework for generalized anisotropy.

The main aniso_pol() function is largely the same as water_pol(),
but without water-specificity.  Makes use of a new type of topology
directive called [anisotropic_polarization], which is not yet
fully integrated.  Will be fully updated in a future commit.

Change-Id: I3f2693f568e2b75493bba30f4c3fc5f450efa3b5