list2df converts a list with components of vector to a data
frame. Shorter vectors will be filled with
NA. It is useful to convert rugged vectors into a data frame which can
be written to an Excel file.
un.list collapses higher-depths list to 1-depth list.
This function uses recursive programming skill to tackle any depths
of list.
shrink.list removes all NULL or NA entries from a list.
## See examples of function feat.mfs for the usages of list2df and un.list.a <- list(x=1, y=NA, z=NULL)
b <- list(x=1, y=NA)
c <- list(x=1, z=NULL)
shrink.list(a)
shrink.list(b)
shrink.list(c)