The nushellx default (imode=0) is that "a"=n or p which ever
has the largest number of valence neutrons nn or protons np.
Bill Rae says that Nushellx runs faster this way.
But with imode=0 one cannot do charge exchange when
np.gte.nn for one of the nuclei.
The Shell default (imode=1) fores "a"=n always
and charge exchange always works. To change the value make a
file called imode.dat and put 0 or 1 in the first line.
