Added a iterator like interface to the data on NDS::buffer
Added cbegin<>()/cend<>() with templated support for selecting the actual data type used.
Reworked the SWIG bindings of NDS::buffer to remove a friend statement, including removing the internal swig_buffer_helper class.
Removed an unused private variable from NDS::buffer.
Note to Ed: I'm mapping std::complex onto the NDS::channel::DATA_TYPE_COMPLEX32 data for a buffer. Is this ok?