[AGIPD][LPD] Add DataFile.create_legacy_source

Description

With the change of source name for corrected data while maintaining backwards compatibility through the concept of legacy sources, this adds a correspondig call creating the soft links to DataFile.

How Has This Been Tested?

with DataFile(p, 'w') as f:
    f.create_index(train_ids)
    
    instr_src = f.create_instrument_source('AN/ACTUAL/DEVICE:output')
    instr_src.create_key('data.bar', np.random.rand(len(train_ids)))
    instr_src.create_index(bar=np.ones(len(train_ids)))
    f.create_legacy_source('A/LEGACY/DEVICE:xtdf', 'AN/ACTUAL/DEVICE:output')
    
    f.create_metadata()

(and pending CI)

Types of changes

• Refactor (refactoring code with no functionality changes)

Reviewers

@ahmedk