eg. VImage::width() is now
int VImage::width() const;
ie. it does not alter the image objects. In factr we can mark almost all
members const.
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