CST_MultiEOF development treat spatial NAs

Hi @erifarov

the function CST_MultiEOF doesn't accept spatial NAs and it'd be very useful if it could accept s2dv_cubes with NAs (e.g. tos has no data over land) and ignore the spatial points with NAs in the calculation (e.g. s2dv::EOF can do it provided that the NAs are consistent along time_dim; but s2dv::EOF is not multivariate as CST_MultiEOF, that's why it'd be useful to have the same functionality also in CST_MultiEOF and be able to compute the MultiEOF (not just EOF) in fields with spatial NAs).

Here is an example of how to call CST_MultiEOF using data with spatial NAs (I just modified the example in the documentation by adding 2 lines to introduce NAs):

seq <- 1 : (2 * 3 * 4 * 5 * 6 * 8)
mod1 <- sin( 0.7 + seq )^2 + cos( seq ^ 2 * 1.22  )
dim(mod1) <- c(dataset = 2, member = 3, sdate = 4, ftime = 5, lat = 6, 
               lon = 8)
mod1[,,,,1,3:4] <- NA # introducing some spatial NAs in the data
mod2 <- sin( seq * 2 ) ^ 3 + cos( seq ^ 2 )
dim(mod2) <- c(dataset = 2, member = 3, sdate = 4, ftime = 5, lat = 6, 
               lon = 8)
mod2[,,,,1,3:4] <- NA # introducing some spatial NAs in the data
lon <- seq(0, 35, 5)
lat <- seq(0, 25, 5)
exp1 <- list(data = mod1, coords = list(lat = lat, lon = lon))
exp2 <- list(data = mod2, coords = list(lat = lat, lon = lon))
attr(exp1, 'class') <- 's2dv_cube'
attr(exp2, 'class') <- 's2dv_cube'
d = as.POSIXct(c("2017/01/01", "2017/01/02", "2017/01/03", "2017/01/04", 
                 "2017/01/05", "2018/01/01", "2018/01/02", "2018/01/03",
                 "2018/01/04", "2018/01/05", "2019/01/01", "2019/01/02", 
                 "2019/01/03", "2019/01/04", "2019/01/05", "2020/01/01", 
                 "2020/01/02", "2020/01/03", "2020/01/04", "2020/01/05"))
exp1$attrs$Dates = d
exp2$attrs$Dates = d

result <- CST_MultiEOF(datalist = list(exp1, exp2), neof_composed = 2)

#Error in CST_MultiEOF(datalist = list(exp1, exp2), neof_composed = 2) :
#  Input data contain NA values.

Do you think you could look into this and see if the development is feasible?

Please let me and @nperez know if you have doubts or need help with it. Thanks a lot!