$im = readenvi( filename ); # read image data
($im, $hdr) = readenvi( filename ); # read image data and hdr data hashref
readenvi will look for an ENVI header file named filename.hdr
If that file is not found, it will try with the windows
convention of replacing the suffix of the filename by .hdr
If valid header data is found, the image will be read and
returned, with a ref to a hash of the hdr data in list
NOTE: This routine only supports raw binary data at this time.