Fix Zero Pattern Issue

[ax3l]

Follow-up to #1118. The pattern issue returned 😱

• add demonstrator: [JSON] Failed opening a file '../samples/varying_pattern_0.json'
• fix Pattern Problem Returns in 0.14.3 #1173

[franzpoeschel]

Inconsistent iteration paddings are correctly read, but they are currently not stored after first parsing.
When accessing an iteration after parsing, it needs to be opened again, using its filename. Currently, that filename is then reconstructed on the fly by using the iteration padding which is wrong.
I've added a commit that stores the filenames and uses the stored ones in favor of reconstructed ones.

[ax3l]

Thank you so much, that's awesome :)

[ax3l]

To apply this fix (especially the third commit) on the release-0.14.5 branch, we problably also need to backport first:

• Apply new frontend design to Container and deriving classes #1115
• Follow-up to redesign of frontend classes: Documentation and cleanup #1159

[ax3l]

Ok I think I managed to backport this in #1282 now 🎉