Add one "previous" version for set.mm and iset.mm

[expln]

The problem: if set.mm (or iset.mm) gets modified by adding some correct change which cannot be processed by mm-lamp, then mm-lamp will fail loading it. This may break all examples based on these files.

This could be improved by the following:

• add set.mm:YYYYMMDD and iset.mm:YYYYMMDD to the list of available sources. They should keep some previous state of the databases mm-lamp can parse and load.
• whenever mm-lamp detects an error related to processing set.mm or iset.mm then it shows a message advising to use set.mm:YYYYMMDD or iset.mm:YYYYMMDD instead, so users are not blocked at least.

The process of loading of the fallback version could be automated as well.