Revert "Initial conditions grid step independent."

Reverts openpixi/openpixi#78

There was some compilation failure:
[INFO] Compilation failure
openpixi/pixi/src/main/java/org/openpixi/pixi/physics/InitialConditions.java:[29,39] cannot find symbol
symbol  : class EmptyPoissonSolver
location: package org.openpixi.pixi.physics.fields