15，matlab中的mean函数，求平均

（1） help mean

mean   Average or mean value.
    S = mean(X) is the mean value of the elements in X if X is a vector. 
    For matrices, S is a row vector containing the mean value of each 
    column. 
    For N-D arrays, S is the mean value of the elements along the first 
    array dimension whose size does not equal 1.
 
    mean(X,DIM) takes the mean along the dimension DIM of X.
 
    S = mean(..., TYPE) specifies the type in which the mean is performed, 
    and the type of S. Available options are:
 
    'double'    -  S has class double for any input X
    'native'    -  S has the same class as X
    'default'   -  If X is floating point, that is double or single,
                   S has the same class as X. If X is not floating point, 
                   S has class double.
 
    S = mean(..., MISSING) specifies how NaN (Not-A-Number) values are 
    treated. The default is 'includenan':
 
    'includenan' - the mean of a vector containing NaN values is also NaN.
    'omitnan'    - the mean of a vector containing NaN values is the mean 
                   of all its non-NaN elements. If all elements are NaN,
                   the result is NaN.
 
    Example: If X = [1 2 3; 3 3 6; 4 6 8; 4 7 7];
 
    then mean(X,1) is [3 4.5 6] and mean(X,2) is [2; 4; 6; 6]
 
    Class support for input X:
       float: double, single
       integer: uint8, int8, uint16, int16, uint32,
                int32, uint64, int64

（2） 例子：