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.

SpMat.h

@@ -93,10 +93,10 @@ class SpMat
 {
 public:
   SpMat() : _m(0), _n(0), _nz(0), _ri(0), _val(0), _pw(false), _nt(1) {}
-  SpMat(unsigned int m, unsigned int n, Utilities::NoOfThreads nt=Utilities::NoOfThreads(1)) : _m(m), _n(n), _nz(0), _ri(n), _val(n), _pw(false), _nt(nt._n) {}
-  SpMat(unsigned int m, unsigned int n, const unsigned int *irp, const unsigned int *jcp, const double *sp, Utilities::NoOfThreads nt=Utilities::NoOfThreads(1));
-  SpMat(const NEWMAT::GeneralMatrix& M, Utilities::NoOfThreads nt=Utilities::NoOfThreads(1));
-  SpMat(const std::string& fname, Utilities::NoOfThreads nt=Utilities::NoOfThreads(1));
+  SpMat(unsigned int m, unsigned int n, Utilities::NoOfThreads nt=Utilities::NoOfThreads()) : _m(m), _n(n), _nz(0), _ri(n), _val(n), _pw(false), _nt(nt._n) {}
+  SpMat(unsigned int m, unsigned int n, const unsigned int *irp, const unsigned int *jcp, const double *sp, Utilities::NoOfThreads nt=Utilities::NoOfThreads());
+  SpMat(const NEWMAT::GeneralMatrix& M, Utilities::NoOfThreads nt=Utilities::NoOfThreads());
+  SpMat(const std::string& fname, Utilities::NoOfThreads nt=Utilities::NoOfThreads());
   ~SpMat() {}
 
   unsigned int Nrows() const {return(_m);}