plug-ins: auto-detect HGT variants.
Since SRTM-1 and SRTM-3 data have a fixed image size, it is actually very easy to auto-detect these by checking the file size. There is no need to ask the user to select between the 2 variants (even though these values are made quite obvious by official download links). I still left the dropdown appear optionally if the detection fails for some reason (we never know, at least that makes a fallback, for instance to be able to load partial data!). Yet now by default, HGT file settings should be fully auto-detected.