spacetime bugs

Hi Edzer,

I got some errors in subsetting STSDF-objects by giving a scalar index for either time or space. I 
attached a self-contained example script that generates the errors (at least it did for me) and a 
modified version of STSDF-methods.R from the latest spacetime-source from CRAN that fixed the issues 
for me. I tested it by running the same example script and hope I didn't miss to test something 
important. I commented changes in the source file with "# TG" at the end of the comments.

regards,
Tom

Technische Universit?t M?nchen
Department f?r Pflanzenwissenschaften
Lehrstuhl f?r Gr?nlandlehre
Alte Akademie 12
85350 Freising / Germany
Phone: ++49 (0)8161 715324
Fax:   ++49 (0)8161 713243
email: tom.gottfried at wzw.tum.de
http://www.wzw.tum.de/gruenland
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