OneShiftOnly should abort when cost function is provided

The OneShiftOnly optimizer minimizes only the total energy and is not capable of variance minimization, or minimization of mixtures of energy and variance.

Additionally, the default cost function (0.9`*`energy+0.1`*`unreweightedvariance) is used by OneShiftOnly for reporting purposes to alert the user if the variance component is getting out of hand (reported cost function in log output is different than just pure total energy, but this a feature intended by @ye-luo and so not a bug in itself).

Due to the above, OneShiftOnly should abort if any cost function is provided by the user, with a message explaining that it 1) cannot accept cost function input and 2) only performs energy minimization.