Fix bug in how compare_results.jl prints missing files

[RuaridhMacd]

When comparing two results folders, files which were missing in one of the folders were being printed in one line, instead of as a list. This fixes that.

[RuaridhMacd]

I've accidentally also included the Julia 1.8 fix here as well.

The changes relevant to this PR are in compare_results.jl

[Betristor]

This is gonna to be very helpful, but what does the output file look like.

[RuaridhMacd]

I've attached an example output below. In comparing two results directories, the comparison reports:

1. Which files / subdirectories are missing from one directory or another.
2. Of the files which are in both, which files match or are mismatched.
3. It then repeats this recursively for any subdirectories.

summary.txt