Ion density sampled evaluation

[dreamer2368]

This PR implements ion density evaluation on sampled grid points, for hyper-reduction of right-hand side of Hartree potential equation.

• Potentials::evalIonDensityOnSamplePts, Potentials::initializeRadialDataOnSampledPts evaluate ion density on specified sampled grid indices.
• While Potentials rescales by rescaleRhoComp and then adds background charge by addBackgroundToRhoComp, ROM rescaling can be done only after hyper-reduction, thus these operations are skipped in this test.