element.py: Only assert __stage_sources_directory is None on initial shell
Description
It's plausible that you'd exit & failed buildshell & then wish to re-enter it within the same context / interrupt dialogue. If we cache that the element has previously been 'shelled' into, we remove the need to assert that the staged sources directory hasn't already been cached when launching into the shell
Changes proposed in this merge request:
- Add new
__previously_shelled
member to Element(), which is toggled bystage_sources()
This merge request, when approved, will close: #1249 (closed)
Edited by Tom Pollard