emd算法中的边界条件函数
%BOUNDARY_CONDITIONS_EMD　extends an extrema set to limit edge effects on the interpolations(% [TMIN,TMAX,ZMIN,ZMAX] = BOUNDARY_CONDITIONS_EMD(INDMIN,INDMAX,T,X,Z,NBSYM)
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% inputs:
%　 - INDMIN, INDMAX: indices of minima and maxima in the real signal X
%　 - T: sampling times
%　 - X: real signal in which INDMIN and INDMAX are the indices of extrema
%　 - Z: signal which values are interpolated in the final envelope
%　 - NBSYM: number of points added to each end
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% outputs:
%　 - TMIN, TMAX: extended sampling times
%　 - ZMIN, ZMAX: extended "extrema" set)