feff6 should incorp F90 "derived types" (structures)

[newville]

F90's derived types (data structures) should be used to reduce the need for common blocks (especially those that span "modules") and to more readily convey complicated data:

1. polarization data
2. potentials info (ph(,,) + il0, etc)
3. concept of Atom (iz, ipot, x, y, z, label)
4. concept of Path (reff, rnorman, deg, array of Atoms, complex k, complex chi)