added a readme document that explains how to install virtualbox on windows, and how to set up a virtual machine
slight change to the IDL_PATH environment variable
deleted a bunch of output files from visualization-output
added a atan2 function I found on the internets ---original atan2 was missing
added a function that can delete a variable ---needed when importing global constants
updated readstationlist function, first item was always empty
(1) function no longer imports all variables from the common block, but stops when it has everything it needs. Variables that are not used are set to 'undefined' using a new function by that name; (2)  there were a couple weirdnesses with variable 'maxz': if maxz was not defined, it was assigned the value 1.0e10. Later on, the variable's type is changed to LONG, but 1.0e10 does not fit a LONG, so this raised a warning. However, the warning was of no real consequence, since the variable was ignored anyway, since the (invalid) conversion result was assigned to a new variable maxl (not maxz). maxl, however, was not used anywhere. I circumcented the problem by assigning maxz 1.0E9 instead of 1.0e10.
(1) deleted the local variable RADIUS, instead I'm now importing from the COMMON block; (2) Now outputting to the visualization-output directory; (3) deleted the call to PSTRACKER
(1) deleted the hardcoded station name. Now using READSTATIONLIST() instead; (2) changed the date, since there was no test data for 20110915 22:00, but there was for 20110815 00:00; (3) deleted the psfile, instead this is now constructed only inside map_ppi(); (4) deleted the call to PSFILE()
removed multi_ppi_ori.pro, since it was doing much the same thing as multi_ppi.pro