Bug fixed: grid to FLT & IMG

Fixed a bug in export grid to FLT, reported by Coen Bussink. The nodata (null) values were not exported correctly (in relation to what was written to the HDR file). He also noted that when exporting to IDRISI the .doc file always had the default output name (= the name of the input grid) even if the .img filename was changed. Version 5.2.0.4