Bulk solvent correction

these papers were influential for the implementation in phenix.refine

Jiang, J.-S. & Brunger, A. T. (1994). J. Mol. Biol. 243, 100-115. "Protein hydration observed by X-ray diffraction. Solvation properties of penicillopepsin and neuraminidase crystal structures."

A. Fokine & A. Urzhumtsev. Acta Cryst. (2002). D58, 1387-1392. "Flat bulk-solvent model: obtaining optimal parameters"

P.V. Afonine, R.W. Grosse-Kunstleve & P.D. Adams. Acta Cryst. (2005). D61, 850-855. "A robust bulk-solvent correction and anisotropic scaling procedure"