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

@@ -59,7 +59,7 @@ public:
                   const std::vector<ExtrapolationType>& et,
                   unsigned int order=3,
                   bool copy_low_order=true,
-                  Utilities::NoOfThreads nthr=Utilities::NoOfThreads(1),
+                  Utilities::NoOfThreads nthr=Utilities::NoOfThreads(),
                   double prec=1e-8,
                   bool data_are_coefs=false)
     : _valid(false), _own_coef(false), _coef(0), _cptr(0), _ndim(0), _nthr(nthr._n)
@@ -71,7 +71,7 @@ public:
                   ExtrapolationType et=Zeros,
                   unsigned int order=3,
                   bool copy_low_order=true,
-                  Utilities::NoOfThreads nthr=Utilities::NoOfThreads(1),
+                  Utilities::NoOfThreads nthr=Utilities::NoOfThreads(),
                   double prec=1e-8,
                   bool data_are_coefs=false)
     : _valid(false), _own_coef(false), _coef(0), _cptr(0), _ndim(0), _nthr(nthr._n)