Make the *SV-based mappers add default values from the schemas

[fontanka16]

No description provided.

[fontanka16]

Proposal to fix this:

• Add a parameter to the get_prop function, for the default value from the map.
• Add a check in (the super().) get_prop that returns this value if no other value has been set.
• In all the places of mapping_file_mapper_base where get_prop is called, add the default parameter or the empty string to the new parameter
• Make sure all implementations of get_prop accepts the parameter
• Remove existing handling for this outside of get_prop in mapping_file_mapper_base. It is handled in at least one place.