Add file handling to proost

Add file handling to proost:

• New Import command
• Ability to launch proost with files.
• detection of cycles in the importation graph
• pretty printing of errors in imported files.

Needs #19 (closed).

added Doing label

assigned to @aalbert

created branch 27-add-file-handling-to-proost to address this issue

mentioned in merge request !26 (merged)

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closed with commit dc1fc185

mentioned in commit dc1fc185

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