Add a salt concentration term to the custom GB forces and add an optional
"kappa" keyword to the Amber createSystem method call to allow users to specify
it.
kappa -> implicitSolventKappa
Allow implicitSolventKappa to have units (it has dimensions of length) and
convert the number to pure nanometers before using it in the GB code.
Merge branch 'master' of https://github.com/SimTk/openmm
Oops, 1/length, not length.
Unit-ify the 0 in implicitSolventKappa.
Merge pull request #249 from swails/master

Add a salt concentration term to the custom GB forces