ENH: Allow `Splinterpolator` instances to be created from an existing set of spline coefficients, and extend extrapolation options

• The Splinterpolator class can now accept a pointer to an array of pre-calculated coefficients - in this case, a copy of the pointer is used (the data is not copied), and pre-filtering (coefficient calculation) is skipped
• New SoftZeros extrapolation method - out of bounds coefficients will be set to zero, but out-of-bounds coordinates will still be constructed via interpolation. This differs from the existing Zeros extrapolation method, where out-of-bounds coordinates are forced to zero.
• Adjustment to the coefficient initialisation method used when Constant extrapolation is requested; previously, a mirroring boundary condition was used.
• New Splinterpolator::Copy function, which allows the spline coefficients to be copied/extracted.

splinterpolator.h

@@ -837,7 +837,12 @@ unsigned int Splinterpolator<T>::get_start_indicies(const double *coord, int *si
   unsigned int ni = _order+1;
 
   for (unsigned int i=0; i<_ndim; i++) {
-    sinds[i] = std::ceil(coord[i]) - ni/2;
+    if (odd(ni)) {
+      sinds[i] = std::floor(coord[i] + 0.5) - ni/2;
+    }
+    else {
+      sinds[i] = std::ceil(coord[i]) - ni/2;
+    }
   }
 
   for (unsigned int i=_ndim; i<5; i++) sinds[i] = 0;