Provide fast splint in OneDimCubicSpline for PBC Coulomb Potentials

Since commit 1bc275373377871b921de37a661ac98ad5197340, splint calls in evaluateSR in PBC Coulomb potentials use cubic spline interpolation. This leads a substantial slowdown, 6% of NiO-a512 calculations, on SPR on Aurora.

diff --git a/src/Numerics/OneDimCubicSpline.h b/src/Numerics/OneDimCubicSpline.h
index f58245e12..004d7b0cc 100644
--- a/src/Numerics/OneDimCubicSpline.h
+++ b/src/Numerics/OneDimCubicSpline.h
- return
- m_grid->cubicInterpolateFirst(m_Y[Loc],m_Y[Loc+1],m_Y1[Loc],m_Y1[Loc+1]);
**Additional context**
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Since the potential grid spacing is small, the cubic spline is not necessary for the accuracy. I understand the change was made to handle a wide range of grids and forces. A possible solution is to add splint_fast.