对有缺测的数据求和

用法：

y=csum(x,k,c)

x为有缺测的数据，如仪器观测数据

k为对第几维求和

c为控制条件，如c为50，表示有50个以上的观测时才求和，否则为nan；如c为-5，表示缺测在5次以下才求和，否则nan-calculate the statistical sum of data but tick off nan

Usage:

y=csum(x,|k) or y=csum(x,k,|c), k can be []

[m,n,...]=size(x)

　if k==1, [1,n,...]=size(y)　

　　　if not set k, then k=1, but if m==1, k=k+1,>>...

　if k==2, [m,1,...]=size(y)

　if k>2, [m,n,...,1(k demension),...]=size(y).

　　you can squeeze(y) by yourself to remove the dim(whose size(y,dim)==1)

c can control output to be nan ro sum, c = 0 by default(namely, not work)

　if c>0, if valuable numbers(notnan) < c, then output nan instead of sum

　if c<0, if nan numbers >= c, then output nan instead of sum