Maps from sub-map sources

[gbiggs]

Relates to #392.

Enhance the map server to allow loading multiple smaller maps and serving them as a single larger map.

An example use case is an occupancy grid for a multi-building complex (like a hospital) where each building (or even each floor?) is maintained as an individual map (image) file, and these are loaded at runtime and served as a single occupancy grid map.

A possible extension or related feature is to allow requesting bits of a map on demand for navigating in extremely large areas.

[SteveMacenski]

This is still in scope for how to:

• deal with very very large maps we need to shard for memory conservation
• support multiple map sources for multi-story buildings or several maps for different areas.

[SteveMacenski]

#1226 supersedes