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

@@ -64,8 +64,7 @@ public:
                   ExtrapolationType et=Zeros,
                   unsigned int order=3,
                   bool copy_low_order=true,
-                  Utilities::NoOfThreads
-                  nthr=Utilities::NoOfThreads(1),
+                  Utilities::NoOfThreads nthr=Utilities::NoOfThreads(1),
                   double prec=1e-8,
                   bool data_are_coefs=false)
     : _valid(false), _own_coef(false), _coef(0), _cptr(0), _ndim(0), _nthr(nthr._n)
@@ -84,6 +83,22 @@ public:
   Splinterpolator& operator=(const Splinterpolator& src)
   { if(_own_coef) delete [] _coef; assign(src); return(*this); }
 
+  // Copy the spline coefficients into dest.
+  void Copy(std::vector<T>& dest)
+  {
+    unsigned int N = 1;
+    for (auto d : _dim) {
+      N *= d;
+    }
+    dest.resize(N);
+
+    auto coefs = coef_ptr();
+
+    for (unsigned int i = 0; i < N; i++) {
+       dest[i] = coefs[i];
+    }
+  }
+
   // Set new data in Splinterpolator.
   void Set(const T *data_or_coefs,
            const std::vector<unsigned int>& dim,