All we are interested in is catching potential exceptions indicating the
variable does not exist, the print was just a sanity check.

This commit changes the way the Tee in Pronterface works. When no log is
specified, sys.stdout is redirected as before to a tee between the
console panel and the true sys.stdout. When a log is specified,
sys.stdout is untouched and logging output is saved in both the console
panel and the log file.
The side effect is that all output must now go through the logging
module to properly get in the console panel and in the log.
Now we have to catch all "print" statements =)

Also cleanup a minor detail in stl plater

This reverts commit a2c0d7a4.

We used to have our own gcode analyzer in gviz but this was clearly
suboptimal and broken. Position of drawn things should not differ in
anyway from the original GCode anyway.

Use precmd() method instead of adding our own layer of preprocessing