/* A C++ version of Matlab's qamdemod(y, M, varargin)
　　*　Align each symbol to the nearest constellation points by minimum Euclidean distance.
　　*　The function only consider rectanglular constellation diagram and grey code.
　　*　The input is assumed to have been downconverted into baseband.
　　*　I is the I component of phase and Q is the Q component of phase.
　　*　The I and Q should be NxM or 1xM matrix/vector, in which the N indicate the subchannels and M indicates the symbols.
　　*　The function treats each row as a subchannel and normalizes them respectively.
　　*/