More compact format for MGF files (fixes #890)

Added an option to store MGF in a more compact manner (omitting zero-intensity peaks and limiting the number of decimal digits). This is now used in MascotAdapterOnline and can be selected in FileConverter with the "MGF_compact" flag.
style: can you change it to 
```
if (intensity == 0.0) 
{
  continue; // skip zero-intensity peaks
}
```