For consistency with the other modules (Python, Ruby and JS), allow including
<std_complex.i> directly when using C# too instead of including <complex.i>
which only works in C++ mode (i.e. when using std::complex) anyhow with C# for
now.
Define csvar{in,out} typemaps needed to support properties of complex type and
apply the existing cstype and csin ones to them as well.
Add unit test verifying that this works as expected in C# and, also, in
Python, even though no changes were needed there.
