Suggestion: simple sanity checks upon attempting to read a VLSV file

[ykempf]

After a few cases of waiting for Godot in production runs where a restart file was not complete, I'd suggest that some simple test be added to check that the xml footer is present at the end of the file. When trying to open a truncated file with Vlasiator, it just hangs. I suppose that just looking whether the file is long enough or whether the last bytes correspond to the end of the xml footer is not too difficult to add?

@sandroos, if you don't have time to implement it, let us know if you have hints and we can look where/how to implement that.