Always save custom content data to file

[ErikDanielsson]

Updated custom_content to always write the data to file. I couldn't figure out how to handle heatmap data however, since it is given as a nested list rather than a dictionary. Should close #1194 eventually.

• This comment contains a description of changes (with reason)
• CHANGELOG.md has been updated

[ewels]

I couldn't figure out how to handle heatmap data however, since it is given as a nested list rather than a dictionary

Yeah I've hit this before with heatmaps. Also some modules create more complicated data structures that don't fit into .tsv files. I think it's fine - the export works fine if using -k yaml or -k json and I've thought about saving all of these output formats anyway.

[ewels]

Ahh, it actually crashes with the heatmap with tsv files.. Ok then we just skip those then.

[ewels]

Ok and whilst this PR fixes the second part of the issue - saving the files even when not exporting the plots - it doesn't solve the first part. The random filenames. Now we get two versions of all the data when running with --export, a random filename and a nice filename..

[ewels]

Ok, done I think. No duplicate files for custom content now, and plot IDs set so no more random alphanumeric sequences.