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

@@ -1643,11 +1643,17 @@ double Splinterpolator<T>::SplineColumn::init_fwd_sweep(double z, ExtrapolationT
     double z2i=z;
     for (unsigned int i=1; i<n; i++, ptr--, z2i*=z) iv += z2i * *ptr;
   }
-  else {
+  else if (et == Mirror) {
     double *ptr=&_col[1];
     double z2i=z;
     for (unsigned int i=1; i<n; i++, ptr++, z2i*=z) iv += z2i * *ptr;
   }
+  // et == Constant || et == Zeros
+  else {
+    double *ptr=&_col[0];
+    double z2i=z;
+    for (unsigned int i=0; i<n; i++, ptr++, z2i*=z) iv += z2i * *ptr;
+  }
   return(iv);
 }
 
@@ -1674,9 +1680,14 @@ double Splinterpolator<T>::SplineColumn::init_bwd_sweep(double z, double lv, Ext
     }
     iv /= (z2i-1.0);
   }
-  else {
+  else if (et == Mirror) {
     iv = -z/(1.0-z*z) * (2.0*_col[_sz-1] - lv);
   }
+  // et == Constant || et == Zeros
+  else  {
+    iv = z / (z - 1) * _col[_sz-1];
+  }
+
   return(iv);
 }