This function accepts a list of Spectra
objects and outputs one STIDF object summarizing
spatial and temporal variation of the input dataset.
If rowSimplify="none", length of the output object will be equal to the sum of all rows of
all elements of the input list object.
If rowSimplify="spc.colMeans", length of the output object will be equal to the number of rows of
the input list object.
This option returns the measurement nearest to the average time of each element of the input list.
firstRow and lastRow : length of the output object equals the number of rows of the input list object.
These two options return the first and last measurements of the input list element