Raise ValueError for off-grid nucleus in clamped_molecular_potential_Poisson

[haakoek]

Raise error for off-grid nucleus in clamped_molecular_potential_Poisson; add force parameter

Previously, if a nuclear radius did not coincide with a radial grid point, the function silently issued a warning and proceeded with the nearest grid point. This is potentially dangerous: the user may not notice the warning, and the result will be inaccurate without any indication of how large the error is.

Changes

• By default (force=False), a ValueError is now raised when a nuclear radius is not a grid point, halting execution and prompting the user to either adjust their grid or explicitly opt in to the approximation.
• Passing force=True restores the previous behaviour: a UserWarning is issued and the nearest grid point is used.
• The existing test for the off-grid warning case was updated to pass force=True and match the new warning message.

Migration

Code that previously relied on the silent fallback should add force=True to the call, or ideally adjust the radial grid so that nuclear positions coincide with grid points.