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

@@ -1436,16 +1436,19 @@ void Splinterpolator<T>::assign(const Splinterpolator<T>& src)
 template<class T>
 bool Splinterpolator<T>::calc_coef(const T *data_or_coefs, bool copy, bool data_are_coefs)
 {
-  if (_order < 2 && !copy) { _cptr = data_or_coefs; return(false); }
+  // No copy, and nearest/interp, or pre-calculated
+  // coefficients - just take a pointer to the data
+  if (_order < 2     && !copy) { _cptr = data_or_coefs; return(false); }
+  if (data_are_coefs && !copy) { _cptr = data_or_coefs; return(false); }
 
   // Allocate memory and put the original data into _coef
-  //
   unsigned int ts=1;
   for (unsigned int i=0; i<_dim.size(); i++) ts *= _dim[i];
   _coef = new T[ts];
   memcpy(_coef,data_or_coefs,ts*sizeof(T));
 
-  if (_order < 2) return(true);  // If nearest neighbour or linear, that's all we need
+  if (_order < 2)     return(true); // If nearest neighbour or linear, that's all we need
+  if (data_are_coefs) return(true); // User has given us pre-calculated coefficients
 
   // Loop over all non-singleton dimensions and deconvolve along them
   //