call Gravity_bhInitFieldVar( integer(in) :: gpotVar )
Called before the tree walk. Initializes fiels variables befor the potential is calculated. Sets index of the grid variable where the gravitational potential is stored (passed from Grid_solvePoisson - ipotvar. Calculate gravitational acceleration at each grid cell (inverted maximum allowed error in acceleration stored in field variable ACEI) - needed for normalization of the error.
gpotVar : index of the grid variable where the gravitational potential is stored. Passed from Grid_solvePoisson.