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

@@ -1350,16 +1350,8 @@ T Splinterpolator<T>::coef(int *indx) const
       case SoftZeros:
       case Zeros:
         return(static_cast<T>(0));
-      case Constant:
-        indx[i] = 0;
-        break;
-      case Mirror:
-        indx[i] = 1-indx[i];
-        break;
-      case Periodic:
-        indx[i] = _dim[i]+indx[i];
-        break;
       default:
+        indx[i] = indx2indx(indx[i], i);
         break;
       }
     }
@@ -1368,23 +1360,17 @@ T Splinterpolator<T>::coef(int *indx) const
       case SoftZeros:
       case Zeros:
         return(static_cast<T>(0));
-      case Constant:
-        indx[i] = _dim[i]-1;
-        break;
-      case Mirror:
-        indx[i] = 2*_dim[i]-indx[i]-1;
-        break;
-      case Periodic:
-        indx[i] = indx[i]-_dim[i];
-        break;
       default:
+        indx[i] = indx2indx(indx[i], i);
         break;
       }
     }
   }
+
   // Now make linear index
   unsigned int lindx=indx[_ndim-1];
   for (int i=_ndim-2; i>=0; i--) lindx = _dim[i]*lindx + indx[i];
+
   return(coef_ptr()[lindx]);
 }