Add support for std::complex<> to C#

Create Lib/csharp/complex.i with simplest possible typemaps mapping
std::complex<> to System.Numerics.Complex.

Lib/csharp/complex.i

@@ -0,0 +1,44 @@
+#ifndef __cplusplus
+#error C# module only supports complex in C++ mode.
+#endif
+
+%{
+#include <complex>
+%}
+
+namespace std {
+
+// An extremely simplified subset of std::complex<> which contains just the
+// methods we need.
+template<typename T>
+class complex
+{
+public:
+    complex(T re, T im);
+
+    T real() const;
+    T imag() const;
+};
+
+} // namespace std
+
+%define swig_complex_typemaps(T)
+%typemap(cstype) std::complex<T>, const std::complex<T>& "System.Numerics.Complex"
+
+// The casts in "pre" are needed in order to allow creating std::complex<float>
+// from System.Numerics.Complex, which always uses doubles. It relies on the
+// fact that the name of the C++ and C# float/double types are the same.
+%typemap(csin,
+         pre="    var cpp$csinput = new Complex_" #T "((" #T ")$csinput.Real, (" #T ")$csinput.Imaginary);"
+        ) std::complex<T>, const std::complex<T>& "Complex_" #T ".getCPtr(cpp$csinput)"
+
+%typemap(csout, excode=SWIGEXCODE) std::complex<T>, const std::complex<T>& {
+    Complex_##T cppret = new Complex_##T($imcall, $owner);$excode
+    return new System.Numerics.Complex(cppret.real(), cppret.imag());
+  }
+
+%template(Complex_##T) std::complex<T>;
+%enddef
+
+swig_complex_typemaps(double)
+swig_complex_typemaps(float)