Provide a writer for a map / double map

[zambrovski]

Scenario

If I want to write to an existing map (instead of building a map on the fly with the builder) there is no current support from the library.

Current Behaviour

Currently, only a builder for the map is available.

Wanted Behaviour

A writer should be available for writing to:

• one map (local and global will end up in the same map)
• two maps (local and global are separate)

Possible Workarounds

Implement the writer on the fly.