Add polarizability prescription - a1 term

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Implement all possible polarization prescription, at least those that are
easy to do (DGF, LAK, a1-term, etc.). Think about other existing
possibilities for -scat and -int options.

Original issue reported on code.google.com by yurkin on 26 Nov 2008 at 8:17

• Blocked on: #106

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Original comment by yurkin on 23 Aug 2010 at 5:27

• Changed title: Add polarizability prescriptions

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DGF and LAK are implemented  by r983. So only 'a1-term' is left to do.

r983 - d2bcb6e159e2f65ccce0c1624c481ae193c8007f

Original comment by yurkin on 2 Oct 2010 at 2:34

[myurkin]

Another idea is to implement Peltoniemi (PEL) formulation. It is a simple modification of the DGF one. Previously, it could be considered a part of the SO formulation, however, the latter was discarded while still in development phase in favor of the fully-analytic IGT(SO) formulation.