all overloads are declared const now, where possible also revised cpp examples and ran them to check see https://github.com/jcupitt/libvips/issues/983
We had exception specs on the C++ interface, but C++11 does not support them and some compilers have never honoured them. Remove all specs. Thanks Lovell. See https://github.com/jcupitt/libvips/issues/362
to cplusplus, see https://github.com/jcupitt/libvips/issues/230
