Remove functions.o from Makefile
anisotropy: Add CIJ_pitl()

This routine calculates ecs for periodic thin layering of isotropic layers
according to
    Postma (1955), Wave propagation in a stratified medium,
    Geophysics, 20(4), 780-806, doi:10.1190/1.1438187

Also add program to deliver constants from command line.
anisotropy: Improve source documentation
anisotropy: Remove unnecessary 'implicit none's from routines.

Fortran modules which have an 'implicit none' statement before the
'contains' apply this to every contained routine.